Add the ability to randomly generate label colors

[correctmost]

Feature Description

I add a lot of labels to my projects. It would be useful if Gitea had the ability to randomly generate a color for new labels to make them visually distinct.

When creating a new label on GitHub, the UI seems to generate a new hex code by default. There's also a dedicated button that allows you to generate a new random hex code.

Screenshots

GitHub's UI

[wxiaoguang]

-> Improve project & label color picker and image scroll #34971