Improve custom css options

CiviCRM currently provides 2 ways to add custom css files, both are problematic:
1) Specify a custom css path in admin->urls. The problem with this is that doing so always disables civicrm.css, which is rarely desirable.
2) Edit the extras.css file in the civicrm core directory. The problem with this is it will get overwritten during upgrades.

Proposed solution: Add a separate option for disabling core css independent of the custom css path setting. Deprecate the use of extras.css