This introduces a new "Follow" menu to the top-right of the pages on

Review Board, alongside "Support". It provides a menu that links to our

news feed, Twitter, Facebook, Reddit, and YouTube channel. The idea is

to get users who may use Review Board but not be aware of any updates,

announcements, or discussions around it to have an easy way to find out

about them and follow them.
For those who really don't want this enabled (for instance, if it

clashes with an internally-defined menu that might confuse users), this

menu can be disabled through settings.DISABLE_FOLLOW_MENU.
This change also introduces a long-overdue CSS modification to make the

menus left-aligned instead of right-aligned. The right-aligned was a

style choice in the rewrite of the header bar UI that really didn't work

for anything more than a couple small items per menu. We already

override this in RBCommons, and with the ability to have custom things

in these menus, left-aligned just makes more sense.

Tested all the links in the menu.