In Review Board 2.5, we introduced the beginnings of mobile UI support.

That implementation was pretty simplistic. We had mixins for defining

rules when in mobile mode (defined as a viewport or screen width of <=

720px), or for showing/hiding when in mobile mode.
This implementation was pretty naive. It wasn't fine-grained enough to

handle the ways in which the UI would break at various viewport sizes,

and made bad assumptions about what constituted a mobile experience.

These ultimately meant that on a mobile phone, things were fine, and at

a larger desktop window, things were fine, but in-between the UI could

look inconsistent or bad.
This change introduces some newer mixins designed to apply rules at

various viewport sizes. Rather than any global device width constants,

callers are expected to specify a size that makes sense for its place in

the UI. For instance, the UI shell (top bar, search, sidebar, etc.) may

optimize better for a particular size (based on the point in which the

search bar would begin to overlap the "Review Board <version>" label,

and the sidebar's display may be based on that exact same width, but a

configuration form that normally aligns field inputs alongside labels

may instead choose to alter its UI based on a separate viewport width

(as it has less room to work with, when a sidebar is present).
The new mixin functions (.on-width-gt(), .on-width-gte(),

.on-width-lt(), and .on-width-lte()) take a viewport width, a

ruleset, and an optional "else" ruleset, for convenience. These live in

the #rb-ns-ui.screen namespace.
The base page also provides a mixin function for specifying rules when

the UI shell is in mobile or desktop mode (which also now occurs at a

width of 800px, and not 720px, to prevent overlapping). These are

.on-shell-desktop-mode() an .on-shell-mobile-mode() in

#rb-ns-pages.base. These are intended for pieces of UI that are

dependent on the exact mode the UI is in (the sidebar's display mode,

for example).
At this time, nothing (including the sidebar) makes use of the UI shell

helpers. An upcoming change to the sidebar will take advantage of this,

though.

Tested the Review Board UI in mobile/desktop modes.
Made use of these mixins in some upcoming changes.