* Fix URI of repeat follow requests not being recorded

In case we receive a “repeat” or “duplicate” follow request, we automatically
fast-forward the accept with the latest received Activity `id`, but we don't
record it.

In general, a “repeat” or “duplicate” follow request may happen if for some
reason (e.g. inconsistent handling of Block or Undo Accept activities, an
instance being brought back up from the dead, etc.) the local instance thought
the remote actor were following them while the remote actor thought otherwise.

In those cases, the remote instance does not know about the older Follow
activity `id`, so keeping that record serves no purpose, but knowing the most
recent one is useful if the remote implementation at some point refers to it
by `id` without inlining it.

* Add tests