Cleanup sheriff-o-matic builder handling.

1. Create CTBuilderFailure for offline/hung bot failure instead of
shoehorning them into CTFailure.
2. Don't use CTBuilderList for ct-builder-failure-card since there's
only ever one builder.
3. Get rid of the builders column on ct-step-failure-card. This is a
UI we inherited from garden-o-matic that I don't think makes as much
sense in the current UI.
4. Rename a bunch of things for consistent naming. Use "step-failure"
for what were previously called builder or sheriff failures. Use
"builder-failure" for the offline/hung bots.

R=leviw@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/569513002

git-svn-id: svn://svn.chromium.org/blink/trunk@182355 bbb929c8-8fbe-4397-9dbb-9b2b20218538