Add unique constraint for Almanac network names

Summary:
The name of networks should be unique.

Also adds support for exact-name queries for AlamanacNetworks.

Test Plan: Applied migration with existing duplicates, saw networks renamed, attempted to add duplicates, got a nice error message.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D19379