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/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)
 	}