This is a follow-up to e6299a3. I additionally fixed these three things:
1. Since e6299a3 there's no need anymore to join the group_users table
when looking for users who were reached by a group mention, so
I removed that join in that commit. But turned out we were joining
the group_users table twice, so I removed the second join in this PR.
That drastically speeded up my test query, from 6 sec to 0.26 sec.
2. We also were joining twice the user_chat_channel_memebership table,
so I removed the second unnecessary join too.
3. We actually need to join the user_chat_channel_memebership table
only in certain cases, and we don't need to do that for group mentions,
so I fixed that too.
As a result of these changes, time of my test query fall down from
6 sec to 0.001 sec. And the resulting SQL query now contains only
one JOIN statement.
a32fce9e1d