Sometimes a broad distinction of just a channelID or a userID
is not enough to efficiently send a websocket event to users.
In several cases, depending on the user and channel, we might
need to modify the message. Therefore, we introduce the
concept of a channel hook that will get executed if the scope
is set to a channel. This hook can be populated at the app layer
to perform any application specific logic to the event.
Care must be taken to avoid race conditions as the passed event
is not deep copied. It is left to the user to treat it carefully.
For this issue, the main problem was that since we don't know
which users have permissions to which channels, we had to go through
_all_ members of a channel to figure that out. This was redundant
since a large portion of those users might not even be connected
at that time.
We solve this with the channel hook where we push this check
to be performed later while actually sending the event. This
reduces the computation to be done only for _connected_ users
rather than _all_ users of a channel.
The next iteration of this should be to use websocket subscriptions
to monitor exactly which users are on that channel to even
trim down that list. That is a larger initiative to be taken later.
Tested locally with a channel of 50 users. Here are rough results:
```
With PR:
patchPost 97ms
createPost 90ms
Master:
patchPost 306ms
createPost - 298ms
```
https://mattermost.atlassian.net/browse/MM-42810
```release-note
Improve performance while sending messages
with permalinks to channels with large number of users.
```
b20ef95b91