Previously, we were incrementing mentions one-by-one
all concurrently in an unbounded fashion.
This would cause a big spike in memory usage if there
were an `@all` mention in a large channel.
We fix this by changing the SQL query to take all userIDs
at once.
https://mattermost.atlassian.net/browse/MM-41752
```release-note
NONE
```
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>