Add follow_request notification type (#12198)

* Add follow_request notification type

The notification type already existed in the backend but was never pushed
to the front-end. This also means translation strings were also available
for the backend, from the notification mailer.

Unlike other notification types, these are off by default, to match what
I remember of Gargron's view on the topic: that follow requests should not
clutter notifications and should instead be reviewed at the user's own
leisure in the dedicated column.

Since follow requests have their own column, I've deemed it unnecessary to
add a specific tab for them in the notification quick filter.

* Show follow request link in single-column if there are pending requests, even if account isn't locked

* Push follow requests from notifications to the follow_requests list

* Offer to accept or reject follow request from the notification

* Redesign follow request notification