Add build.collection_id column

Summary:
The plan is to use the collection_id to tag all builds generated by a
single BuildIndexAPIView/PhabricatorNotifyDiffAPIView request.
Later, when builds finish, we will use the collection_id to figure out
if the finishing build is the last one in its collection, in which
case we will send out an email with results for all builds in that
collection.

Test Plan:
Ran alembic upgrade head.
Verified (using psql) that the correct column and index were created in the build table.
Ran alembic downgrade -1 to make sure the migration can be reverted.
Verified that the collection_id column and index were gone.
Ran alembic upgrade head again.
Ran py.test tests/ with no errors.

Reviewers: cf, ar, vishal

Reviewed By: cf, vishal

Subscribers: cramer, reviews-dfbgrowth, christoffer

Maniphest Tasks: T25400

Differential Revision: https://tails.corp.dropbox.com/D79833