app: remove generated assets after installation
app: remove generated assets after installation

Flask-Assets uses Python's webassets to rebuild static assets, and
webassets will not rebuild them in static/gen if the are more
recent than the installed files. Which typically happens if:

* package is built at time T
* install package built at time T
* package is built at time T+1
* use installed packages for the first at time T+2 which generates
  assets in static/gen dated T+2
* install package built at time T+1
* the generated assets are more recent (T+2) than the installed
  files sources (T+1) and will not be rebuilt and therefore reflecting
  the content of the package built at time T instead of the content of
  the package built at time T+1

This is a rare condition which is only likely to show when testing
release candidates

heartsucker/securedrop,heartsucker/securedrop,garrettr/securedrop,garrettr/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop