Every review request (and review-request like box such as the file review UI)

has a header that includes some icon-based actions, a bunch of text-based

actions, and some tabs. The code for these was spread out over every view and

template that wanted to include them, and the functionality was slightly

different for every implementation.
This change centralizes all of that into a single template which renders all of

the actions (including the "archive" menu, which was previously only available

on the main review request page). The tabs are handled somewhat specially by

passing in a list of tab data into the template context. The logic for the

"current tab" is done by comparing the request path to the tab's URL.

Checked the header display and functionality on the main review request page,

the diff viewer page (with different diff revisions), and the image review UI.