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/app/channel.go

@@ -1834,6 +1834,20 @@ func (a *App) GetChannels(c request.CTX, channelIDs []string) ([]*model.Channel,
 	return channels, nil
 }
 
+func (a *App) GetChannelsMemberCount(c request.CTX, channelIDs []string) (map[string]int64, *model.AppError) {
+	channelsCount, err := a.Srv().Store().Channel().GetChannelsMemberCount(channelIDs)
+	if err != nil {
+		var nfErr *store.ErrNotFound
+		switch {
+		case errors.As(err, &nfErr):
+			return nil, model.NewAppError("GetChannelsMemberCount", "app.channel.get_channels_member_count.existing.app_error", nil, "", http.StatusNotFound).Wrap(err)
+		default:
+			return nil, model.NewAppError("GetChannelsMemberCount", "app.channel.get_channels_member_count.find.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
+		}
+	}
+	return channelsCount, nil
+}
+
 func (a *App) GetChannelByName(c request.CTX, channelName, teamID string, includeDeleted bool) (*model.Channel, *model.AppError) {
 	var channel *model.Channel
 	var err error