This introduces new styles for representing tabular data on a review

request in a field, along the lines of the Issue Summary Table. This

will allow us to provide consistency with some of the new bits of

tabular data we're introducing (the commits list, primarily).
The styles support displaying filtering tabs (like the issue states) and

other filtering widgets above the table. It's up to consumers to

actually hook any of this up, though.
This is the first new set of styles that make use of a proposed internal

standard for improving the organization of both the LessCSS rules and

CSS class names. Each of the CSS classes has a namespaced name, starting

with rb-c-review-request-field-tabular, which indicates 1) that it's

a Review Board CSS class and 2) that it's a component (c). All

classes within that use this prefix to ensure we don't run into any

namespace collisions within the product or with extensions.
On top of this, we use a -modifier-name format for CSS classes that

affect the presentation/behavior of a style, helping differentiate these

sorts of modifier rules from standard, independent rules.
LessCSS variables are also now namespaced and stored along with the new

rules in a new file. The namespacing gives us a cleaner way of defining

and organizing variables. Splitting these off into a new file helps us

organize the rules, which shouldn't have much of a penalty with the

modern way we generate and bundle files.
Upcoming changes will move the Issue Summary Table and Commit List to

use these styles as a base.

Tested this along with an upcoming change that makes use of the new

styling. This was tested with all functionality provided by the style,

in desktop and mobile modes.