Make `<hr>` and .form-control higher contrast

With light on a screen, the contrast is too low to see the form fields.
Add a small amount of contrast to aid.
Enforce valid hostnames (minus uppercase) for app IDs

Marathon’s modified hostnames allow digits, dashes, dots, and lowercase
letters. Normal hostnames allow uppercase letters and are case
insensitive, but that would likely be awkward in Marathon's app naming.
Allow assets to be built from any directory.
Merge pull request #321 from mesosphere/build-assets-from-any-directory

Allow assets to be built from any directory.
Document app ID requirements in REST.md

App IDs must match `/^[a-z0-9.-]+$/` currently, and that should be
documented.

Fixes #314.
Update ID regex to current form in REST docs
Expose app versions in UI

* Add model and collection for AppVersion
* Expose all versions in the “Configuration” tab
* Roll back to previous versions via a button on the version row
Rename JSX files to .jsx for new compilation setup