Link to deleted user profile page available to users from reaction

[trichoplax]

Describe the bug
If a user adds a reaction to a post and that user is then deleted, the reaction still shows their username, and clicking on it allows navigating to their profile page. To a user, this profile page will contain no information, but this is inconsistent with everywhere else that a deleted username shows.

Expected behavior
The reaction should show "deleted user" instead of the username, and there should be no link to the user profile for standard users.

A link may be required for moderators - see #1416

[cellio]

Let's remove reactions upon account deletion. The whole point of a reaction is to make a public, attributed assertion -- "who says so?" is an important part of the feature.

[trichoplax]

Is there likely to be account reinstatement in future? I can't see a way to do it through the interface at present, but if in future it becomes possible, would we want old removed reactions to reappear? If we remove them by setting a flag rather than deleting the database row, then reactions will be free to return in future if that's ever decided to be desired.

[cellio]

That's a good point. Withholding (in the UI) is better than deleting the data. (I don't know if a UI "is deleted?" check is fine or if we should add a "hide?" column to the relevant table, but either way, let's preserve the data in case it's needed later.)