Add privileges

Fixes #1177 (closed)
FE part for pleroma!3676 (closed)

We can now check what privileges someone has besides only the role. On older back-ends, privileges aren't provided, so we have a fall-back to list the privileges corresponding to what mods/admins are allowed on older BE's.

• add privileges
• only show options when privileged
  • delete post from other user
  • See moderation menu for users (Related !1454 (closed))
    • Grant Admin
    • Grant Moderator
    • Deactivate account
    • Delete account
    • User tags (I think the rest are user tags, I should double check)
  • See/don't see deactivated accounts and moderation menu on them
• test that it still works with an old be. I believe admins have all privileges, mods have statuses_delete, but I should double check
• Check other MR's where this may be relevant
  • Being able to handle reports (merged by now): !1322 (merged)
    • BE shouldn't send these notifications when they aren't privileged in the first place, but there's the "interactions" which also has a tab for reports. This tab shouldn't show if not privileged.
  • Announcements (At time of writing not yet merged): !1466 (merged)
    • There was already a comment for it, I linked back to here. If !1466 (merged) gets merged before this, I should do the changes here.