We were invalidating the userProfileByIds cache unnecessarily
when it wasn't really needed. It's just that the InvalidateCacheForUser
method wiped off all user related caches, when we just needed to wipe
user-channel related caches.
To fix this, we split the user cache invalidation method into two
and just call the InvalidateChannelCacheForUser method from app/channel.go.
https://mattermost.atlassian.net/browse/MM-59827
```release-note
NONE
```