This feature allows @ mentions to prioritize showing members of a group who
have explicit permission to a category.
This makes it far easier to @ mention group member when composing topics in
categories where only the group has access.
For example:
If Sam, Jane an Joan have access to bugs category.
Then `@` will auto complete to (jane,joan,sam) ordered on last seen at
This feature works on new topics and existing topics. There is an explicit
exclusion of trust level 0,1,2 groups cause they get too big.
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