+addrelease form doesn't validate uniqueness of (distribution, name)

Steps to reproduce:

1. http://localhost:8086/distros/ubuntu/+addrelease
2. Fill in distribution information:
   - Name: hoary
   - Parent Release: The Warty Warthog Release
   - Other fields: anything
3. Crash:

ProgrammingError: ERROR: duplicate key violates unique constraint "distrorelease_distribution_key" INSERT INTO DistroRelease (id, datereleased, displayname, description, messagecount, lucilleconfig, driver, distribution, releasestatus, parentrelease, owner, datelastlangpack, changeslist, name, title, sourcecount, binarycount, summary, version, nominatedarchindep) VALUES (13, NULL, 'test distro', 'a test distribution to show that this form lacks input validation', 0, NULL, NULL, 1, 1, 1, 16, NULL, NULL, 'hoary', 'a test distribution', DEFAULT, DEFAULT, 'test distribution', '1.0', NULL)