Add blocks for the admin templates needed for upcoming Djblets changes.

Review Request #8789 — Created March 1, 2017 and submitted

Christian Hammond
Review Board
There's some work being done for Djblets 0.10 that adds better structure
and reusability for the administration-related templates. One of the
things that change does is creates standardized blocks for admin media
that doesn't rely on Review Board's templates so that they can be used
in a standard Django install.

However, given the blocks that Review Board overrides or completely
leaves out, those changes do not immediately work. This change updates
our base admin template to define a block that can be used by child
templates and ensures we call the parent block for "extrahead", fixing
some templates.

Tested with the upcoming Djblets change. Saw that all media was properly
loading on all the pages.

Review Bot
  1. Tool: Pyflakes
    Ignored Files:
    Tool: PEP8 Style Checker
    Ignored Files:
David Trowbridge
  1. Ship It!
Christian Hammond
Review request changed

Status: Closed (submitted)

Change Summary:

Pushed to release-3.0.x (07dcedd)