This gives the diff collapse button a fresher look, helping it to fit in

with the new diff viewer styles. It's now the same shade of blue as the

diff headers, has a slightly rounded look, and says "Collapse" rather

than just showing an icon.
This button has been converted to a CSS component, and the HTML in the

templates de-duplicated, helping to keep this all manageable. It's also

been given ARIA attributes and a role to help screen readers (although

there are still usability challenges with the DOM placement of this

button). Further improvement for specifying line numbers and the

filename for screen readers would be helpful as well, but is a larger

task.
The z-index of the button has been fixed so it won't float above the

Unified Banner.
Some of the old CSS around this button have been cleaned up.
The button icon is handled by mixing in a .rb-icon-... definition.

This required partially reverting a recent change to make all

definitions private in LessCSS. The reason for that issue before was

that two mixins were named .icon(), and thus were combining into one

rule. Since both have been renamed, this is no longer a problem. To

further help avoid issues, .rb-icon() has been renamed to

.make-rb-icon().

Unit tests pass.
Tested the collapse button in the diff viewer in various kinds of

expanded content, and in diff fragments for changed, added, and

deleted files. Verified positioning and behavior in each.
Verified that there was no issues between datagrid and Review Board

icons.