Move colorItems to ItemList

[imorland]

Changes proposed in this pull request:
At present, if an extension or theme wishes to add/remove any items from Colors, dom traversal is required.
To allow for extensions/themes to better integrate with the Appearance Page, each element under Colors is now built within an ItemList.

Screenshot
Visibly, no changes

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[SychO9]

if an extension/theme is adding a new color variable, shouldn't that be in the extension's own settings page 🤔 ?

[imorland]

A couple of potential use cases:

• fof/nightmode may want to remove the dark mode toggle, as this might be confusing to have that active/available when "auto night mode" is active via that extension

• A possible future extension could replace the text input for color hex with a color picker

[askvortsov1]

A few thoughts before merging.

I'm fine with this change for now, but I want to see a big revamp of the admin dashboard as part of 2.0. To the point where we might want to revert some of the changes we made in v1.0. Some particular concerns:

• It's not clear whether extensions should be putting settings on their own pages, or extending other extension pages / core pages.
• Some extensions clearly need their own pages, others really don't. There's no visual indication in the UI which is which.
• Some extensions are really headless plugins rather than full-fledged modules, it would be nice to distinguish between the two.