This eases build setup. Where Yarn requires a separate package
repository, npm is available in the standard one.
yarn's package repository signature was recently expired, and apparently
will expire again in a year. Avoid dealing with that.
Fixes#110.
Besides being more clear about what belongs to which, this helps with
docker caching. The server and ui parts are only rebuilt when their
respective subdirectories change.
Extend this a bit further by making the webpack build not depend on
the target architecture. And adding cache dirs so parts of the server
and ui build process can be reused when layer-wide caching fails.
