Make topics a "tag/label" system instead of a "category" system

[JelteF]

Often patches fall in multiple of the topics. So we should replace it with a many-to-many system as opposed to a many-to-one relationship. We should probably also rename it to "tag" instead of "topic". These changes would allow us to add many more useful labeling, like "good-first-review".

[JelteF]

@jchampio do you wanna assign yourself here?

Also for future reference, some of my thoughts on this from a thread on discord:

1. don't duplicate entries on the page
2. add a tags column
3. I'd be fine with doing a hard removal of categories if the tags feature has filtering support (the category feature is really underdesigned). But going for incremental additions seems fine too.
4. Should we call them tags or labels?
5. We probably (eventually) want colors for tags. Tag coloring sounds like a great usecase for this oklch (it's a semi-new color type CSS) thing I learned about recently: https://oklch.com/#70,0.1,140,100 People could pick a "hue" and maybe a "chroma" and you have colors that are all the same lightness so.

[jchampio]

@JelteF I think I need collaborator permission first, but yes.

[JelteF]

Apparently it was enough for you to respond here for me to be allowed to add you as the assignee.

[polobo]

A tag/label feature is a high priority for me but I wouldn't expect or try to replace the existing Topic model; though tweaking the available topics is on the table. I would go with tags but I'd be pressed to explain why.

[JelteF]

Fixed by #69