ui: review notification categories

[miguelgrc]

Feature request

Currently the frontend lets you create as many notification categories as you want with any arbitrary name. However, the backend only allows you to create notifications only inside of the "publish" and "review" categories. We should rethink this so that it makes more sense from the UI PoV.

What is the expected behavior? The notifications UI is more clear about what notification categories are allowed.

What is motivation or use case for adding/changing the behavior? The UI is now confusing, since you can create any category but when creating a notification you can get a 400 error.

How should this be implemented in your opinion? We either show those two categories always and by default even if they're empty (and disallow creation of new ones) or we only let people choose between "publish" and "review" when creating new categories. I prefer the first option but we would have to "hardcode" that (or update the schemas) since now we're taking the categories from the notifications schema, meaning that new schemas won't have any category.

If only those two categories are allowed, the UI could be slightly restructured: we can have a single page with all the notifications and two collapsible sections, one with "publish" notifications and one with "review" notifications, since it doesn't make that much sense to have a sidebar just for two categories.

Are you willing to work on this yourself? yes