Add the ability to ban people

[monorkin]

When running a public instance sometimes there is a need to ban someone - for spam, trolling, or any other reason.

Only an administrator should be able to ban someone.
They shouldn't be able to ban themselves or another administrator.

Banning someone should:

• deactivate their account
• keep their email unchanged
• delete all memberships
• delete all push subscriptions
• delete all searches
• delete all sessions
• prevent them from using a session transfer code
• the ban should be undoable

Optionally:

• expunge their conversations with a placeholder message (e.g. [Removed by admin]) and delete all their boosts
  • undoing a ban, of-course, doesn't undo the expunge
  • the expunge can be an additional option when banning someone
• an optional reason for the ban can be provided

[milos-dukic]

Maybe to add that the action to ban/un-ban user needs like a pop-up dialog to confirm the actions, with the note that the user messages and boosts will be deleted for good, memberships revoked etc.?

[milos-dukic]

Also it would be nice, to group the users which are banned, maybe at the top, so you don't have to scroll through the whole list of users to find banned one, i.e. management of the banned used could be easier with grouping.

[milos-dukic]

PR #95