Re-add submodule

Deploy failed without it.

[hqdb0-staging.internal.commcarehq.org] out: clearing resource cache
[hqdb0-staging.internal.commcarehq.org] out: OSError: [Errno 2] No such file or directory: '/home/cchq/www/staging/Vellum'
Merge branch 'multimedia-case-properties' into autostaging
Merge remote-tracking branch 'origin/cache-locations' into autostaging
Merge remote-tracking branch 'origin/hq-dropdown' into autostaging
Merge remote-tracking branch 'origin/refactor-domain-copy2' into autostaging
Merge branch 'vellum-requirejs' into autostaging
Merge remote-tracking branch 'origin/dynamic-links' into autostaging
Merge remote-tracking branch 'origin/offline-install-multimedia' into autostaging
Merge remote-tracking branch 'origin/logos' into autostaging
Merge remote-tracking branch 'origin/109909-case-performance' into autostaging
Merge remote-tracking branch 'origin/log-telerivet-calls' into autostaging
Merge remote-tracking branch 'origin/switch_to_site_code' into autostaging
update submodule refs