when remote follows local, hold off sending Accept if remote identity not fully fetched

[alphatownsman]

When a new remote identity follows a local identity (manual approval = false), Follow.by_ap will create a Follow and a bare bone Identity, so these will be triggered:

• IdentityStatus.handle_outdated
• FollowStates.handle_unrequested
• FollowStates.handle_accepting

there are rare cases FollowStates.handle_accepting get executed before IdentityStatus.handle_outdated, which will raise exception bc missing inbox_uri

This PR should fix this.

[andrewgodwin]

Thanks!