No way to clean up plugin configs

**Is your feature request related to a problem? Please describe.**
When working on a plugin, I ended up in a state where there was no application mapped to it. Notifications came through as `/unknown` and no combination of removing or reinstalling the plugin would reinitialize it.
Similarly, deleting the plugin channel from 'apps' leaves it in a weird state. (Possibly the same as above.)

**Describe the solution you'd like**
There should be a way to purge old information and reset a plugin configuration from scratch.
Gotify should display old plugin configurations even when the plugin is missing.

**Describe alternatives you've considered**
I ended up hacking on the db manually.

Start by tracing how plugin configurations are stored and how plugin and app/channel removal are handled in the server. Define the reset and purge behavior, then verify that missing-plugin configurations remain visible and that reinstalling or resetting a plugin removes the broken state.

Written by the indexing model from the issue text.