Proper priority management for events

It would be nice to have graded priorities, where one post has priority over another, and both have priority over signups without priority. This would make it much easier for event organisers to manage events. Changes would need to be made to the app and web frontend too.

No files or tests are named. Start by tracing how event posts, signups, and any existing priority data are represented in the app and web frontend; map the relevant entry points before deciding the scope. Done means organisers can distinguish graded priorities consistently across both interfaces.

Written by the indexing model from the issue text.