The Admin V3 styles are now the primary styles for the application. This change promotes the `admin_v3` directory to `admin` and archives the old styles.
Changes:
- Renamed `app/webpacker/css/admin_v3` to `app/webpacker/css/admin`.
- Renamed the previous `app/webpacker/css/admin` to `app/webpacker/css/admin_legacy`.
- Moved all files referenced by V3 styles from the legacy directory to the new `admin` directory.
- Updated imports in `all.scss` to distinct local files instead of relative paths.
- Cleaned up `admin_legacy` by removing files that are duplicates (by name or content) of the new admin styles.
- Updated `admin-style-v3` pack to point to the new location.
There were all sorts of dependencies hiding in there, and it was overwriting some of the admin variables. Better to keep things separate and simple.
I discovered that there was one legitimate reason for importing them though: for styling the trix editor. I can't think of a good way to safely import them without affecting the other variables, so I just hardcoded the colours for now.
