[6.x] Revision improvements

[JayGeorge]

Description of the Problem

We had a few different revision state messages in place, but it didn't quite cover everything.

If you've saved a revision but you haven't published it, it wasn't clear that what you're working on was effectively just a draft/working copy.

More details here

What this PR Does

• Adds a warning message when you've saved changes but haven't published them. Now you get warned "This is a working copy"
• Closes Revisions are confusing #13543

Status Before

(no warning that this is the working copy, when you look at the revisions panel on the right)

Status After

While I was here I've fixed a few other things:

• Warning colors now have AA for contrast
• Dark mode for warning colors
• Corrected the bottom margin so the vertical gap is the same as the horizontal gap. Here's it before:

Clipped focus

See how the blue focus border was clipped on the side. This is now fixed

How to Reproduce

1. Enable revisions for a collection
2. Publish an entry
3. Work on an entry, then save it
4. Before this PR you would simply see the message "Entry has a published version". With this PR you will now see "This is the working copy" if it's not yet been published