Exercise editor: support updates

[mb21]

see https://github.com/achirkin/qua-kit/issues/70

Notes

• enables methodOverride middleware
• Yesod.Helpers.Crud could come in handy for such use cases, though I haven't looked at it closely, since there was existing code for create and read in this case.
• adds a column to the DB.

Questions

• visually, the pages look quite hideous... is that okay for the admin pages, or...?
• ExerciseSettings with key and value columns doesn't appear to be needed since we have the onSubmitMsg and canAddDeleteGeom columns (added by you), as well as the canEditProperties column (added in this pull).

[achirkin]

visually, the pages look quite hideous... is that okay for the admin pages, or...?

That's true :-D It would be nice to make it more usable, but only if we have time for that, since it is only visible to a few admins.

ExerciseSettings with key and value columns doesn't appear to be needed since we have the onSubmitMsg and canAddDeleteGeom columns (added by you), as well as the canEditProperties column (added in this pull).

Yeah.. I added it only because I did not have time for a proper solution :) So I still think, it makes sense to put all settings into another table. Let's keep it in mind, but postpone for a while

[mb21]

okay, then this should be ready to merge.. or should I add some CSS today?