Add new stylesheets for base page and form rules.

The base rules that comprise every page (standard tag styling, header
bar, branding area, etc.) are now split out from common.less into
pages/base.less.

A ui/forms.less has been started with some of the standard form styling.

While tearing parts out of common.less, I found two rules,
`.new-review-request` and `#searchbar`, which were no longer used. The
former was orphaned when RB 2.0's New Review Request page was written,
and the latter was orphaned with the UI rewrite for RB 1.7.

Reviewed at https://reviews.reviewboard.org/r/5864/