Remove all remaining occurences of fakeApp (#17661)

* Remove all remaining occurences of fakeApp

Migrated any remaining methods under server.

And for everything else, renamed fakeApp to app.
The word fakeApp is confusing and we should just call
it for what it is - an app.

https://focalboard-community.octo.mattermost.com/workspace/zyoahc9uapdn3xdptac6jb69ic?id=285b80a3-257d-41f6-8cf4-ed80ca9d92e5&v=495cdb4d-c13a-4992-8eb9-80cfee2819a4&c=639a0bc1-4401-43d5-81ec-0dd54e796d9a

```release-note
NONE
```

* fix tests

app/plugin_requests.go

@@ -20,11 +20,11 @@ import (
 	"github.com/mattermost/mattermost-server/v5/utils"
 )
 
-func (a *App) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
-	pluginsEnvironment := a.GetPluginsEnvironment()
+func (s *Server) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
+	pluginsEnvironment := s.GetPluginsEnvironment()
 	if pluginsEnvironment == nil {
 		err := model.NewAppError("ServePluginRequest", "app.plugin.disabled.app_error", nil, "Enable plugins to serve plugin requests", http.StatusNotImplemented)
-		a.Log().Error(err.Error())
+		s.Log.Error(err.Error())
 		w.WriteHeader(err.StatusCode)
 		w.Header().Set("Content-Type", "application/json")
 		w.Write([]byte(err.ToJson()))
@@ -34,7 +34,7 @@ func (a *App) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
 	params := mux.Vars(r)
 	hooks, err := pluginsEnvironment.HooksForPlugin(params["plugin_id"])
 	if err != nil {
-		a.Log().Error("Access to route for non-existent plugin",
+		s.Log.Error("Access to route for non-existent plugin",
 			mlog.String("missing_plugin_id", params["plugin_id"]),
 			mlog.String("url", r.URL.String()),
 			mlog.Err(err))
@@ -42,7 +42,7 @@ func (a *App) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	a.servePluginRequest(w, r, hooks.ServeHTTP)
+	s.servePluginRequest(w, r, hooks.ServeHTTP)
 }
 
 func (a *App) ServeInterPluginRequest(w http.ResponseWriter, r *http.Request, sourcePluginId, destinationPluginId string) {
@@ -81,7 +81,7 @@ func (a *App) ServeInterPluginRequest(w http.ResponseWriter, r *http.Request, so
 
 // ServePluginPublicRequest serves public plugin files
 // at the URL http(s)://$SITE_URL/plugins/$PLUGIN_ID/public/{anything}
-func (a *App) ServePluginPublicRequest(w http.ResponseWriter, r *http.Request) {
+func (s *Server) ServePluginPublicRequest(w http.ResponseWriter, r *http.Request) {
 	if strings.HasSuffix(r.URL.Path, "/") {
 		http.NotFound(w, r)
 		return
@@ -91,7 +91,7 @@ func (a *App) ServePluginPublicRequest(w http.ResponseWriter, r *http.Request) {
 	vars := mux.Vars(r)
 	pluginID := vars["plugin_id"]
 
-	pluginsEnv := a.GetPluginsEnvironment()
+	pluginsEnv := s.GetPluginsEnvironment()
 
 	// Check if someone has nullified the pluginsEnv in the meantime
 	if pluginsEnv == nil {
@@ -115,11 +115,11 @@ func (a *App) ServePluginPublicRequest(w http.ResponseWriter, r *http.Request) {
 	http.ServeFile(w, r, publicFile)
 }
 
-func (a *App) servePluginRequest(w http.ResponseWriter, r *http.Request, handler func(*plugin.Context, http.ResponseWriter, *http.Request)) {
+func (s *Server) servePluginRequest(w http.ResponseWriter, r *http.Request, handler func(*plugin.Context, http.ResponseWriter, *http.Request)) {
 	token := ""
 	context := &plugin.Context{
 		RequestId:      model.NewId(),
-		IpAddress:      utils.GetIPAddress(r, a.Config().ServiceSettings.TrustedProxyIPHeader),
+		IpAddress:      utils.GetIPAddress(r, s.Config().ServiceSettings.TrustedProxyIPHeader),
 		AcceptLanguage: r.Header.Get("Accept-Language"),
 		UserAgent:      r.UserAgent(),
 	}
@@ -142,7 +142,7 @@ func (a *App) servePluginRequest(w http.ResponseWriter, r *http.Request, handler
 
 	r.Header.Del("Mattermost-User-Id")
 	if token != "" {
-		session, err := a.GetSession(token)
+		session, err := New(ServerConnector(s)).GetSession(token)
 		defer ReturnSessionToPool(session)
 
 		csrfCheckPassed := false
@@ -184,10 +184,10 @@ func (a *App) servePluginRequest(w http.ResponseWriter, r *http.Request, handler
 					mlog.String("user_id", userID),
 				}
 
-				if *a.Config().ServiceSettings.ExperimentalStrictCSRFEnforcement {
-					a.Log().Warn(csrfErrorMessage, fields...)
+				if *s.Config().ServiceSettings.ExperimentalStrictCSRFEnforcement {
+					s.Log.Warn(csrfErrorMessage, fields...)
 				} else {
-					a.Log().Debug(csrfErrorMessage, fields...)
+					s.Log.Debug(csrfErrorMessage, fields...)
 					csrfCheckPassed = true
 				}
 			}
@@ -213,7 +213,7 @@ func (a *App) servePluginRequest(w http.ResponseWriter, r *http.Request, handler
 
 	params := mux.Vars(r)
 
-	subpath, _ := utils.GetSubpathFromConfig(a.Config())
+	subpath, _ := utils.GetSubpathFromConfig(s.Config())
 
 	newQuery := r.URL.Query()
 	newQuery.Del("access_token")