Run client build before database migration

[natefoo]

Your Galaxy server can typically continue to work while an upgrade is in progress before restarting, until either the new client is installed (which might break things) or the database is migrated (which might break things). Because the migrations (typically) don't take too long and the client build can take a while, especially on a slow network filesystem (but nothing is written to the static directory until the end), we could switch the client build to occur before the migration (currently it happens right after) so that the time period that Galaxy might be broken is the shortest.