Add dedicated builder revisions page to sherrif-o-matic

This adds a new /revisions.html page that lists all builders and the
latest revision for each repository for said builder.
Builders are grouped by buildmaster (i.e chromium, chromium.webkit) and
shown in alphabetical order.

R=ojan@chromium.org
BUG=400410
TEST=scripts/test/revisions-tests.html

Review URL: https://codereview.chromium.org/506183003

git-svn-id: svn://svn.chromium.org/blink/trunk@181066 bbb929c8-8fbe-4397-9dbb-9b2b20218538