Feature/cdodge/alpha microsite2

@sefk would you be in a position to give this a quick once over? I still have some cleanup to do and tests to write, but I think this addresses some of your feedback (i.e. having template/.css overrides in a directory outside of edx-platform). I still need to chat with DevOps about how these microsite definition directories get deployed/managed.

Ideally, I need to have the approach locked down next week as the customer should start delivering some of the override assets. 

UPDATE: @ichuang can I get some MITx eyeballs on this as well? Basically this feature is to support separate deployable "micro-themes" (e.g. branding elements, Mako template overrides, etc.) so that we can get a bit of branding "multi-tenancy" on the edx.org production servers (e.g. foo.edx.org, bar.edx.org). My primary concern is regarding preventing regressions with MITx (and Stanford who is looking at this now) instances. Question for your team: Does the MITx instance actually use the legacy SUBDOMAIN_BRANDING and VIRTUAL_UNIVERSITIES settings? Sef @Stanford was asking if - it's not actually used - if we can get rid of it. Thanks.