Move wp-admin.css and rtl.css to the /css folder

[scribu]

All css files should be in one place.

Moves wp-admin.css & rtl.css to /css, updating url() references

If we are going to do this may as well change rtl.css to wp-admin-rtl.css to follow the naming standard. Also should probably do it early in the 3.0 dev. cycle.

They were in the css dir for a brief time but some plugins that accessed them directly broke. That was quite some time ago, so it might not be an issue any more.

so let's put this in sothat it can be tested broadly. Just some hours ago I asked myself as well why that .css file is placed in the admin-root instead of the css folder. +1 to move.

I'll raise the priority a bit because other changes will depend on it.

Related: #11645

Patch in #11561 needs to be updated in case this gets committed.

(In [12553]) Move wp-admin.css and rtl.css to the wp-admin/css directory, rename rtl.css to wp-admin-rtl.css to follow naming standard, fixes #10985