Completes migration to the new admin v3 styling system by:
- Removing conditional stylesheet inclusion in admin head
- Deleting all legacy admin style files and components
- Making admin-style-v3 the default and only stylesheet
This change reduces maintenance overhead and simplifies the admin styling codebase by removing the old styling system that was being conditionally loaded based on feature flags.
It added specificity but had no use.
I reviewed a couple of screens to make sure:
- /admin/orders/Rx/customer
- /admin/properties/new
I have to confess I don't know how Spree::Admin::BaseHelper is included, or where it's used.
Best viewed with whitespace ignored.
Errors and required fields now use the colour from the palette. The error messages in the old design are a bit darker now, but I think it's clearer. And it makes the new design look way better.
Also snuck in a tiny padding tweak.
The current value is 13px already, but I'd like to experiment changing this.
I'm not sure if we even need the reset, but am not delving into that right now.
Errors:
```
SassError: SassError: compound selectors may no longer be extended.
Consider `@extend a, :hover` instead.
See http://bit.ly/ExtendCompound for details.
```
