Update our .browserslistrc
Update our .browserslistrc

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explicitly listing versions that quickly get out of date, we now use the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

Testing Done:
- Ran `browserslist` manually to verify that the config gets parsed into a
  reasonable target browser set.
- Recompiled all static media and ran js-tests.

Reviewed at https://reviews.reviewboard.org/r/11421/

reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard