Get rid of one more jQuery dependant and have a nicer color picker as
well.
Now there is only a single global color picker init because that is all
that's necessary because the elements are present on the page when the
init code runs. The init is slightly weird because the module only takes
a selector instead of DOM elements directly.
The label modals now also perform form validation because previously it
was possible to trigger a 500 error `Color cannot be empty.` by clearing
out the color value on labels.
<img width="867" alt="Screenshot 2024-03-25 at 00 21 05"
src="https://github.com/go-gitea/gitea/assets/115237/71215c39-abb1-4881-b5c1-9954b4a89adb">
<img width="860" alt="Screenshot 2024-03-25 at 00 20 48"
src="https://github.com/go-gitea/gitea/assets/115237/a12cb68f-c38b-4433-ba05-53bbb4b1023e">
dd8dde2be8