Currently missing values will get the value <no value>
rather than the empty string. Using the empty string is
more consistent, and should be easier for users to deal with too.
This is with `golint -min_confidence=0.5`.
I left several lint warnings untouched because they were either
incorrect or I felt it was better not to change them at the moment.
Changes to the UI:
- "Active Since" timestamps are now human-readable.
- Alerting rules are now pretty-printed better.
- Labels are no longer just strings, but alert bubbles (like we do on
the status page for base labels).
- Alert states and target health states are now capitalized in the
presentation layer rather than at the source.
