Add support for gui config on the api

[RikudouSage]

As discussed on Matrix, an endpoint that allows returning and storing arbitrary JSON data that can be used by the GUI would be great.

That would allow:

• storing your synchronization settings across all browsers you're logged in
• storing your filter preference for the various filters in the gui across browsers
• displaying a list of the censures you actually synchronize (including the other instances you fetch your lists from)

Endpoints:

• GET /config/gui - returns the JSON
• POST /config/gui - stores the JSON
• GET /config/gui/{instance} - returns the JSON for the particular instance. I imagine it would be best to have this available on an opt-in basis, meaning if you don't allow displaying this publicly, it would return 404.

Edit: As discussed, 10 kiB of data seems more than enough currently.