[Liam W] (RIP) Mentions Mini Avatars 2021-07-29

This install-and-done add-on will add user avatars to the left of any mentions of them.

This add-on uses the XF2 'embed metadata' system to store the mentioned users with each message. This means that for any message created after this add-on is installed, there will only be one additional query per thread for the user avatars.

For messages created before this add-on is installed, there is an option to control what you wish to happen. This provides three alternatives:

• Always [load legacy mentions]
  This will only show an avatar with pre-addon message mentions, either loading the user from the entity cache, or querying the database for the user if it isn't cached. This can cause an increase in per-page queries, depending on the number of mentioned users that are in the entity cache.
• [Load] Cached only [legacy mentions]
  This will only show an avatar with pre-addon message mentions if the mentioned user is in the entity cache.
• Never [load legacy mentions]
  This will not show avatars for mentions in pre-addon messages.

Note: For this first beta, all messages other than posts will be treated as pre-addon messages, however the metadata is being stored with them for beta 2.