1. For file endpoints, a recent optimization added the ChannelID column
to be part of the fileinfo table. Therefore, we can skip the postID
and directly use the channelID.
2. For post endpoints, we reorder the sequence of calls such that
we get the channelID first, and use it to check the permission of the channel
rather than query the long way around by joining with the posts table
in the permissions query.
The benefit is that SessionHasPermissionToChannel is cache-backed.
So in the happy path, we save a DB call. Because GetSinglePost anyways
needed to be called.
And in the bad path, we replace it with a more efficient call. Because
SessionHasPermissionToChannel is cache-backed, so effectively we
are replacing SessionHasPermissionToChannelByPost with GetSinglePost.
3. And then for the calls that don't have the channelID available,
we change the implementation itself to get the channelID by querying
the posts table first, and then calling SessionHasPermissionToChannel.
This creates the happy path as mentioned earlier.
While here, we also do some other optimizations:
4. Pre-populate the channelID while saving the reaction, so that
we don't need to query the posts table for every single reaction save.
5. Remove unnecessary goroutine spawning for publishing reaction events,
because anyways those are asynchronous.
https://mattermost.atlassian.net/browse/MM-56877
```release-note
NONE
```
Co-authored-by: Mattermost Build <build@mattermost.com>
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