MM-43202: Change some 501 response codes to fix SLO violations (#20767)

There were lots of cases where we were using the code 501
in an incorrect manner, and since 5xx codes are considered
as SLO violations, these were flagged by our monitoring incorrectly.

These were the guidelines to change the codes:
1. Any case of missing license or config not correctly enabled
is set as Forbidden. The logic is that the user is correctly
authenticated with a session, but does not have enough privileges
to have the higher license, or hasn't changed the config correctly.
2. Any case of incorrectly formed request is obviously a 400.

https://mattermost.atlassian.net/browse/MM-43202

```release-note
NONE
```

app/emoji.go

@@ -39,11 +39,11 @@ const (
 
 func (a *App) CreateEmoji(sessionUserId string, emoji *model.Emoji, multiPartImageData *multipart.Form) (*model.Emoji, *model.AppError) {
 	if !*a.Config().ServiceSettings.EnableCustomEmoji {
-		return nil, model.NewAppError("UploadEmojiImage", "api.emoji.disabled.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("UploadEmojiImage", "api.emoji.disabled.app_error", nil, "", http.StatusForbidden)
 	}
 
 	if *a.Config().FileSettings.DriverName == "" {
-		return nil, model.NewAppError("GetEmoji", "api.emoji.storage.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("GetEmoji", "api.emoji.storage.app_error", nil, "", http.StatusForbidden)
 	}
 
 	// wipe the emoji id so that existing emojis can't get overwritten
@@ -100,11 +100,11 @@ func (a *App) GetEmojiList(page, perPage int, sort string) ([]*model.Emoji, *mod
 
 func (a *App) UploadEmojiImage(id string, imageData *multipart.FileHeader) *model.AppError {
 	if !*a.Config().ServiceSettings.EnableCustomEmoji {
-		return model.NewAppError("UploadEmojiImage", "api.emoji.disabled.app_error", nil, "", http.StatusNotImplemented)
+		return model.NewAppError("UploadEmojiImage", "api.emoji.disabled.app_error", nil, "", http.StatusForbidden)
 	}
 
 	if *a.Config().FileSettings.DriverName == "" {
-		return model.NewAppError("UploadEmojiImage", "api.emoji.storage.app_error", nil, "", http.StatusNotImplemented)
+		return model.NewAppError("UploadEmojiImage", "api.emoji.storage.app_error", nil, "", http.StatusForbidden)
 	}
 
 	file, err := imageData.Open()
@@ -185,11 +185,11 @@ func (a *App) DeleteEmoji(emoji *model.Emoji) *model.AppError {
 
 func (a *App) GetEmoji(emojiId string) (*model.Emoji, *model.AppError) {
 	if !*a.Config().ServiceSettings.EnableCustomEmoji {
-		return nil, model.NewAppError("GetEmoji", "api.emoji.disabled.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("GetEmoji", "api.emoji.disabled.app_error", nil, "", http.StatusForbidden)
 	}
 
 	if *a.Config().FileSettings.DriverName == "" {
-		return nil, model.NewAppError("GetEmoji", "api.emoji.storage.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("GetEmoji", "api.emoji.storage.app_error", nil, "", http.StatusForbidden)
 	}
 
 	emoji, err := a.Srv().Store.Emoji().Get(context.Background(), emojiId, true)
@@ -208,11 +208,11 @@ func (a *App) GetEmoji(emojiId string) (*model.Emoji, *model.AppError) {
 
 func (a *App) GetEmojiByName(emojiName string) (*model.Emoji, *model.AppError) {
 	if !*a.Config().ServiceSettings.EnableCustomEmoji {
-		return nil, model.NewAppError("GetEmojiByName", "api.emoji.disabled.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("GetEmojiByName", "api.emoji.disabled.app_error", nil, "", http.StatusForbidden)
 	}
 
 	if *a.Config().FileSettings.DriverName == "" {
-		return nil, model.NewAppError("GetEmojiByName", "api.emoji.storage.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("GetEmojiByName", "api.emoji.storage.app_error", nil, "", http.StatusForbidden)
 	}
 
 	emoji, err := a.Srv().Store.Emoji().GetByName(context.Background(), emojiName, true)
@@ -231,7 +231,7 @@ func (a *App) GetEmojiByName(emojiName string) (*model.Emoji, *model.AppError) {
 
 func (a *App) GetMultipleEmojiByName(names []string) ([]*model.Emoji, *model.AppError) {
 	if !*a.Config().ServiceSettings.EnableCustomEmoji {
-		return nil, model.NewAppError("GetMultipleEmojiByName", "api.emoji.disabled.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("GetMultipleEmojiByName", "api.emoji.disabled.app_error", nil, "", http.StatusForbidden)
 	}
 
 	emoji, err := a.Srv().Store.Emoji().GetMultipleByName(names)
@@ -269,7 +269,7 @@ func (a *App) GetEmojiImage(emojiId string) ([]byte, string, *model.AppError) {
 
 func (a *App) SearchEmoji(name string, prefixOnly bool, limit int) ([]*model.Emoji, *model.AppError) {
 	if !*a.Config().ServiceSettings.EnableCustomEmoji {
-		return nil, model.NewAppError("SearchEmoji", "api.emoji.disabled.app_error", nil, "", http.StatusNotImplemented)
+		return nil, model.NewAppError("SearchEmoji", "api.emoji.disabled.app_error", nil, "", http.StatusForbidden)
 	}
 
 	list, err := a.Srv().Store.Emoji().Search(name, prefixOnly, limit)