After first follow from an instance, resend activities

[fauno]

We talked about this a few times but couldn't find an issue reported.

Related to #101

When you look for an Actor profile on Mastodon, and the Actor doesn't have any previous followers from your instance, Mastodon will fetch the Actor to show a profile, but won't fetch activities. So it shows an empty profile with a tiny message saying you need to go visit the remote profile if you want to read previous activities.

This has been surprising to people testing Sutty + Social Inbox.

But if the Social Inbox resends some activities when a new instance is detected, we can prevent confusing all users from that instance. Only the first one :P

@RangerMauve mentioned they're tracking an issue on Mastodon that could solve this entirely, could you link it here?

[RangerMauve]

This is the main tracking issue on the mastodon side mastodon/mastodon#34

[fauno]

New instance detection means first follower from an unknown instance.

[sutty-coop]

We move this goal from M3 to M4

[fauno]

We need to test if Mastodon will take old activities first

[RangerMauve]

mastodon/mastodon#34