This change updates our browserslist file for Review Board 5.0, and

should hopefully be more future-proof going forward. Instead of

explicitly listing versions that quickly get out of date, we now use the

browserslist "defaults" key, which means the last two versions of every

browser that has > 0.5% market share. We then explicitly disable a few

browsers that we don't want (IE11, Baidu, KaiOS, Opera Mini), leaving us

with recent versions of desktop and mobile browsers which are webkit,

chromium, and gecko-based.

Ran browserslist manually to verify that the config gets parsed into a

  reasonable target browser set.
Recompiled all static media and ran js-tests.