[Suspend] Appeal suspensions functionality

## Feature Request

**Is your feature request related to a problem? Please describe.**
Allow suspended user to appeal their suspensions. This should ideally show similarly to a report to admins.

**Describe the solution you'd like**
Allow suspended users to appeal suspensions a set number of times if they choose.

This could take inspiration from existing functionality in Flarum's reports extension, allowing a user to explain why they should be unsuspended.

Ideally, we'd have an appeal as a discussion that can be replied to multiple times. A decision on the appeal could show as an event post, and integration with flarum/lock could be used to lock the post when a decision has been reached.

**Justify why this feature belongs in Flarum's core, rather than in a third-party extension**
This is a tricky one. Integration into suspend for appeals would be best in the core extension because it's a fairly standard feature that comes alongside bans in other software, but it's entirely possible for it to be a community extension too. This is up to us to decide.

**Describe alternatives you've considered**
We could implement this idea into or steal some code from Byobu if we wanted, as it handles private discussions and such well.

Start by reviewing the existing suspend and reports extension behavior, then compare Byobu's handling of private discussions. Consider how flarum/lock could represent a final decision. Done would require an agreed scope for appeal limits, discussion replies, admin decisions, and whether this belongs in core or a third-party extension.

Written by the indexing model from the issue text.