services/swh-web: Compile static assets with webapp editable install

When swh-web has been installed in editable mode in the swh/stack docker image (when testing changes from a merge request for instance), ensure to compile static assets with webpack as those are only provided in release install of the webapp.

When one wants to test the changes of a swh-web merge request with docker, the line containing swh.web in requirements-swh.txt must be replaced by:

-e git+https://gitlab.softwareheritage.org/swh/devel/swh-web.git@refs/merge-requests/<mr-id>/head#egg=<package_name>