MM-53669: Use the cache layer for EmojiStore.GetMultipleByName (#24030)

There was already a cache present for emoji names. But we weren't using
it for the GetMultipleByName method. Now we implement that method
to look up the cache for every emoji name passed.

Secondly, a bigger problem was that we were making the DB call for system
emojis as well. Since system emojis aren't stored in the DB, it would
fall through the cache layer and always make a redundant DB call. In the
profiles, this should up as taking 16% of the total time to serve
a getPostsForChannel API endpoint.

We fix this by filtering the emojis to only custom emojis
before making the call.

https://mattermost.atlassian.net/browse/MM-53669

```release-note
NONE
```

server/channels/store/timerlayer/timerlayer.go

@@ -3128,10 +3128,10 @@ func (s *TimerLayerEmojiStore) GetList(offset int, limit int, sort string) ([]*m
 	return result, err
 }
 
-func (s *TimerLayerEmojiStore) GetMultipleByName(names []string) ([]*model.Emoji, error) {
+func (s *TimerLayerEmojiStore) GetMultipleByName(ctx context.Context, names []string) ([]*model.Emoji, error) {
 	start := time.Now()
 
-	result, err := s.EmojiStore.GetMultipleByName(names)
+	result, err := s.EmojiStore.GetMultipleByName(ctx, names)
 
 	elapsed := float64(time.Since(start)) / float64(time.Second)
 	if s.Root.Metrics != nil {