Utilize SQLAlchemy to prevent N+1 queries on the deferred description column

[dstufft]

It doesn't exist yet, but when https://gerrit.sqlalchemy.org/#/c/zzzeek/sqlalchemy/+/679/ and https://bitbucket.org/zzzeek/sqlalchemy/issues/3037/support-setting-load-options-on-instances are both done it would be a good idea to utilize them so that accessing the Release.description field without explicitly undefering it would raise an error.

This would prevent us from accidentally accessing it and having SQLAlchemy issue extras queries behind the scenes, causing N+1 queries.