deduplicate bitmask.js

since github doesn't track hard liks, we need to deduplicate bitmask.js library. there should be only one copy of this, we think that ui/app/lib is the proper place.

however, for testing and developing it would be nice to be able to test bitmask.js/rest api against the backend (dummy).

we could have some development flag to serve just bitmask.js library, w/o needing to import the whole bitmask_js UI package. this should be easy since the relative import is always fixed re. core._web

(from redmine: created on 2016-09-28)