MM42267: Add member count in the browse channel modal (#23800)

* add base for calling the endpoint

* add endpoint and handler

* update store and layers

* call the endpoint

* align types

* update app layers

* generate mocks

* complete handler

* finish store query

* add todos

* add ui for member count

* add selector

* add a todo

* add cache layer

* optimize calls in FE

* handle invalidation of the cache

* fix go style

* fix test

* use existing channel layer count

* fix import error

* delete unnecessary code

* write tests for channel cache layer

* fix testname

* fix mocks

* fix cache layer test

* fix a test

* really fix the test

* write more tests for server

* address PR comments

* remove comment

* rename more_channels to browse_channels

* fix style

* update snapshot

* add translations

* Revert "add translations"

This reverts commit 56476a5dabe357703ef02be9a38b3e88f5c8b1e7.

* add only related translations

* address PR review points

* add test

* fix test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/channel.go

@@ -25,6 +25,7 @@ func (api *API) InitChannel() {
 	api.BaseRoutes.Channels.Handle("/group", api.APISessionRequired(createGroupChannel)).Methods("POST")
 	api.BaseRoutes.Channels.Handle("/members/{user_id:[A-Za-z0-9]+}/view", api.APISessionRequired(viewChannel)).Methods("POST")
 	api.BaseRoutes.Channels.Handle("/{channel_id:[A-Za-z0-9]+}/scheme", api.APISessionRequired(updateChannelScheme)).Methods("PUT")
+	api.BaseRoutes.Channels.Handle("/stats/member_count", api.APISessionRequired(getChannelsMemberCount)).Methods("POST")
 
 	api.BaseRoutes.ChannelsForTeam.Handle("", api.APISessionRequired(getPublicChannelsForTeam)).Methods("GET")
 	api.BaseRoutes.ChannelsForTeam.Handle("/deleted", api.APISessionRequired(getDeletedChannelsForTeam)).Methods("GET")
@@ -681,6 +682,29 @@ func getChannelStats(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func getChannelsMemberCount(c *Context, w http.ResponseWriter, r *http.Request) {
+	if c.Err != nil {
+		return
+	}
+
+	channelIDs := model.ArrayFromJSON(r.Body)
+	if !c.App.SessionHasPermissionToChannels(c.AppContext, *c.AppContext.Session(), channelIDs, model.PermissionReadChannel) {
+		c.SetPermissionError(model.PermissionReadChannel)
+		return
+	}
+
+	channelsMemberCount, err := c.App.GetChannelsMemberCount(c.AppContext, channelIDs)
+
+	if err != nil {
+		c.Err = err
+		return
+	}
+
+	if err := json.NewEncoder(w).Encode(channelsMemberCount); err != nil {
+		c.Logger.Warn("Error while writing response", mlog.Err(err))
+	}
+}
+
 func getPinnedPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequireChannelId()
 	if c.Err != nil {