Upgrade web-app component to Rails 4.2

[GUI]

With the release of Rails 5.0, it means Rails 3.2 is no longer supported. Our web-app component that provides the admin and internal API functionality is still on Rails 3.2.

The primary motivator for upgrading as soon as possible is that this lack of support means Rails 3.2 will no longer receive any security updates. That's not to say there are suddenly known security issues with our app, but since any future security issues won't be addressed in Rails 3.2, we need to upgrade.

And the reason for upgrading to Rails 4.2 instead of going directly to 5.0:

• 4.2 will likely be an easier initial upgrade path.
• 5.0 is only a couple weeks old, so at least right now, I think we'll have better luck with gem compatibility under the more established 4.2.
• 4.2 will likely be supported for longer than 5.0 for security releases. As we saw with Rails 3 & 4, Rails 3.2 was actually supported for longer than the 4.0 series, since the Rails maintenance policy means severe security issues will continue to be patched in the last previous major release (so 4.2 will continue to see patches until Rails 6.0 comes out).

As a prerequisite to this task, if we can wait until we pull the admin UI out into a standalone Ember CLI app as part of the Ember 2 upgrade, then I think it will make upgrading the Rails app easier (since we don't have to contend with the older Ember gems, asset pipeline integration, etc).

[GUI]

Landed in #312