Adds a feature flag to control availability of DMs in shared channels (#28920)

* Adds a feature flag to control availability of DMs in shared channels

* Reverse feature flag wording

server/channels/app/channel.go

@@ -424,7 +424,7 @@ func (a *App) createDirectChannel(c request.CTX, userID string, otherUserID stri
 }
 
 func (a *App) createDirectChannelWithUser(c request.CTX, user, otherUser *model.User, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError) {
-	if user.IsRemote() || otherUser.IsRemote() {
+	if !a.Config().FeatureFlags.EnableSharedChannelsDMs && (user.IsRemote() || otherUser.IsRemote()) {
 		return nil, model.NewAppError("createDirectChannelWithUser", "api.channel.create_channel.direct_channel.remote_restricted.app_error", nil, "", http.StatusForbidden)
 	}
 
@@ -528,9 +528,11 @@ func (a *App) createGroupChannel(c request.CTX, userIDs []string) (*model.Channe
 		return nil, model.NewAppError("CreateGroupChannel", "api.channel.create_group.bad_user.app_error", nil, "user_ids="+model.ArrayToJSON(userIDs), http.StatusBadRequest)
 	}
 
-	for _, user := range users {
-		if user.IsRemote() {
-			return nil, model.NewAppError("createGroupChannel", "api.channel.create_group.remote_restricted.app_error", nil, "", http.StatusForbidden)
+	if !a.Config().FeatureFlags.EnableSharedChannelsDMs {
+		for _, user := range users {
+			if user.IsRemote() {
+				return nil, model.NewAppError("createGroupChannel", "api.channel.create_group.remote_restricted.app_error", nil, "", http.StatusForbidden)
+			}
 		}
 	}
 

server/channels/app/post.go

@@ -141,7 +141,7 @@ func (a *App) deduplicateCreatePost(rctx request.CTX, post *model.Post) (foundPo
 }
 
 func (a *App) CreatePost(c request.CTX, post *model.Post, channel *model.Channel, flags model.CreatePostFlags) (savedPost *model.Post, err *model.AppError) {
-	if channel.IsShared() && (channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup) {
+	if !a.Config().FeatureFlags.EnableSharedChannelsDMs && channel.IsShared() && (channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup) {
 		return nil, model.NewAppError("CreatePost", "app.post.create_post.shared_dm_or_gm.app_error", nil, "", http.StatusBadRequest)
 	}