Refresh webfinger (#1323)

* Refresh local info for remote accounts when webfinger returns new values

It only refreshes account info if one of the URLs or the public-key changes,
in which cases it refreshes the full info, re-downloading the feeds from that
user.

Some special handling should probably be done when the public key changes,
but I have been unable to find any use for it in Mastodon yet.

* Re-fetch remote users we aren't subscribed to.

This might induce performance issues, we might want to only do that for users
we explicitly attempted to subscribe but failed to.

* Refactor changes

* Do not refresh existing remote account details more than once a day

* Avoid re-fetching webfinger info in tests unless otherwise specified