Destroy follow requests on exhausted delivery attempts

[ClearlyClaire]

In rare cases, it is possible a follow request never reaches the server of the person that one is trying to follow.

Currently, that failure case is indistinguishable from other failure cases as far as the end-user is concerned, but that failure case can be detected by Mastodon.

In such a case, it might be more useful to delete the follow request than show it as pending, although I am not quite sure how this could be perceived by the user attempting to follow. Ideally, the error should be recorded and displayed to the user, but that requires changes to the API, storage and clients.

Also related: #21956, #21957

[jsoref]

Couldn't you trigger a notification to the user? I get notifications for Follows, Replies, and Favorites. Do you really need to define a new notification type, or could you just use an existing one?

[ClearlyClaire]

Couldn't you trigger a notification to the user? I get notifications for Follows, Replies, and Favorites. Do you really need to define a new notification type, or could you just use an existing one?

No, we can't use an existing notification type for that, we'd need a new one.

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions]

This pull request has resolved merge conflicts and is ready for review.

[ClearlyClaire]

In such a case, it might be more useful to delete the follow request than show it as pending, although I am not quite sure how this could be perceived by the user attempting to follow. Ideally, the error should be recorded and displayed to the user, but that requires changes to the API, storage and clients.

We probably do want to make those changes, and that should probably be through adding a column to follow requests, as well as change the response serializer to embed this information.

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.