[LNT] Implement separate backend server

[Quuxplusone]

Bugzilla Link	PR27534
Status	NEW
Importance	P normal
Reported by	Daniel Dunbar (daniel@zuster.org)
Reported on	2016-04-27 01:32:28 -0700
Last modified on	2016-04-29 13:04:57 -0700
Version	unspecified
Hardware	PC All
CC	chris.matthews@apple.com, daniel@zuster.org
Fixed by commit(s)
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LNT needs to be factored into a front-end and a back-end. This will support:
 - Clearer policies around interactions with the database.
 - Improved caching for results.
 - Opportunities for long-lived processing jobs running in the backend (decoupled from an individual front-end HTTP request).

Given our existing infrastructure, we should probably do this by just
continuing to use Flask to server REST APIs to the front-end.

When running out-of-the-box or in a simple development configuration, we should
just run the backend in process, or in a manner that is cleanly coupled to the
launched process.

When running in a larger scale deployment, administrators should be responsible
for factoring the execution of the front-end and back-end services
appropriately.

[Quuxplusone]

I can take this.