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/storetest/emoji_store.go

@@ -154,26 +154,26 @@ func testEmojiGetMultipleByName(t *testing.T, ss store.Store) {
 	}()
 
 	t.Run("one emoji", func(t *testing.T) {
-		received, err := ss.Emoji().GetMultipleByName([]string{emojis[0].Name})
+		received, err := ss.Emoji().GetMultipleByName(context.Background(), []string{emojis[0].Name})
 		require.NoError(t, err, "could not get emoji")
 		require.Len(t, received, 1, "got incorrect emoji")
 		require.Equal(t, *received[0], emojis[0], "got incorrect emoji")
 	})
 
 	t.Run("multiple emojis", func(t *testing.T) {
-		received, err := ss.Emoji().GetMultipleByName([]string{emojis[0].Name, emojis[1].Name, emojis[2].Name})
+		received, err := ss.Emoji().GetMultipleByName(context.Background(), []string{emojis[0].Name, emojis[1].Name, emojis[2].Name})
 		require.NoError(t, err, "could not get emojis")
 		require.Len(t, received, 3, "got incorrect emojis")
 	})
 
 	t.Run("one nonexistent emoji", func(t *testing.T) {
-		received, err := ss.Emoji().GetMultipleByName([]string{"ab"})
+		received, err := ss.Emoji().GetMultipleByName(context.Background(), []string{"ab"})
 		require.NoError(t, err, "could not get emoji", err)
 		require.Empty(t, received, "got incorrect emoji")
 	})
 
 	t.Run("multiple emojis with nonexistent names", func(t *testing.T) {
-		received, err := ss.Emoji().GetMultipleByName([]string{emojis[0].Name, emojis[1].Name, emojis[2].Name, "abcd", "1234"})
+		received, err := ss.Emoji().GetMultipleByName(context.Background(), []string{emojis[0].Name, emojis[1].Name, emojis[2].Name, "abcd", "1234"})
 		require.NoError(t, err, "could not get emojis")
 		require.Len(t, received, 3, "got incorrect emojis")
 	})

server/channels/store/storetest/mocks/EmojiStore.go

@@ -108,25 +108,25 @@ func (_m *EmojiStore) GetList(offset int, limit int, sort string) ([]*model.Emoj
 	return r0, r1
 }
 
-// GetMultipleByName provides a mock function with given fields: names
-func (_m *EmojiStore) GetMultipleByName(names []string) ([]*model.Emoji, error) {
-	ret := _m.Called(names)
+// GetMultipleByName provides a mock function with given fields: ctx, names
+func (_m *EmojiStore) GetMultipleByName(ctx context.Context, names []string) ([]*model.Emoji, error) {
+	ret := _m.Called(ctx, names)
 
 	var r0 []*model.Emoji
 	var r1 error
-	if rf, ok := ret.Get(0).(func([]string) ([]*model.Emoji, error)); ok {
-		return rf(names)
+	if rf, ok := ret.Get(0).(func(context.Context, []string) ([]*model.Emoji, error)); ok {
+		return rf(ctx, names)
 	}
-	if rf, ok := ret.Get(0).(func([]string) []*model.Emoji); ok {
-		r0 = rf(names)
+	if rf, ok := ret.Get(0).(func(context.Context, []string) []*model.Emoji); ok {
+		r0 = rf(ctx, names)
 	} else {
 		if ret.Get(0) != nil {
 			r0 = ret.Get(0).([]*model.Emoji)
 		}
 	}
 
-	if rf, ok := ret.Get(1).(func([]string) error); ok {
-		r1 = rf(names)
+	if rf, ok := ret.Get(1).(func(context.Context, []string) error); ok {
+		r1 = rf(ctx, names)
 	} else {
 		r1 = ret.Error(1)
 	}