Updates README to include requirement of loading env vars during asset
compilation when deploying to Heroku

[finishes #60689204]
Beautifies administrative views (and makes brand-compliant) by applying PivotStrap.

- Configuration Edit: factored out Build Setup and Tracker Setup into
  separate partials.
- ProjectEdit.init: validates the Feed URL on load (instead of
  checking for build being online); makes the "CI Status" label behave
  as expected (e.g. was showing error when creating a new config, now
  signals "unconfigured")
- includes new gems: bootstrap-sass, pivotal-sass

[finishes #56443842]