BE - Process mentions on annotation creation

[mtomilov]

When an annotation is created, check if it contains mentions and create them appropriately.

The raw process should look close to:

1. Extract terms prefixed with @, as potential username mentions.
2. Check which of them belong to actual users from the group in which the annotation is posted.
3. Pick the first N unique users (We talked about limiting it to 5 mentions per annotation)
4. Replace the @username entries for those with the corresponding @pubid.
5. Create mentions entries for all the mentions created during this process (depends on https://github.com/orgs/hypothesis/projects/153/views/1?pane=issue&itemId=92950044).

[acelaya]

Just to clarify, there's a few outdated points in the issue description, which do not correspond with the latest state of things.

1. Extract terms prefixed with @, as potential username mentions.

This is no longer correct. Only mentions using the special tag format should be eligible for an actual user mention. Any appearance of @whatever which is not wrapped in the special tag should be ignored and considered plain text.

4. Replace the @username entries for those with the corresponding @pubid.

We decided to not use pubids in the end. The user will be identified by their userid, which will be set in the data-userid attribute of the special mention tag.

[mtomilov]

The thread were all the mention filtering logic was discussed

In essense

• limiting number of mentions to 5 by default
• nipsa users can neither send nor receive mentions
• no mentions from private annotations
• mentions allowed only either in world group or if mentioned user belongs to the same group as that of the annotation