Replace coloris with vanilla-colorful (#30201)
Found [a better color
picker](https://github.com/web-padawan/vanilla-colorful) that [does not
rely](https://github.com/mdbassit/Coloris/issues/139) on
`querySelectorAll` or a global shared instance, and is also around a
third of the size of the previous one.
The popover is handled by tippy.js for which I introduced a new "bare"
theme and it uses a new sibling-based mechanism which should prove
useful later to create tippy popovers via HTML only.
<img width="846" alt="Screenshot 2024-03-31 at 04 03 38"
src="https://github.com/go-gitea/gitea/assets/115237/7639b911-a2d7-4f5c-bffd-a9d84561e747">
