We had been throwing around pseudo-constants "past" "current" and "future".
This converts them into an enum and renames "current" to "present" to
disambiguate.

We had been passing only the effective date of a version, but the new design
calls for both a start end end date.

The "Effective" heading hasn't been obvious enough to users, particularly when
reviewing previous versions of the regulation. These template and style tweaks
change that by adding the end date where applicable and highlighting the
version when it's not the current law.

USWDS doesn't have an orange color, so we'll want to remove it eventually. In
the short term, though, we'll deprecate its usage. We can remove with the next
major release.

New Features
* #506 "Effective date" UI now includes more information and warns when not
  viewing the current version.

Bug Fixes & Misc
* #497 Update frontend dependencies.
* #501 Set Sauce Labs key in Travis UI.
* #502 Improve accessibility by including the text of a link in its
  aria-label.
* #503 Fix some HTML escaping issues.
* #504 Fix scrolling issue in regulation timeline.
* #505 Darken Subpart labels to provide more contrast. Remove ids from SVGs
  for automated readers.
* #506 Replaced "orange" values with their "gold" equivalents