cli: add command 'kallithea-cli front-end-build'

Kallithea is under the GPL license, and we can thus only distribute any
generated code if we also ship the corresponding source.

We are moving towards a web front-end that use npm to download and compile
various open source components. The components might not be GPL, but if we
distribute any parts of their code (compiled or converted to other
representation), then we also must distribute the corresponding source under
the GPL.

It doesn't seem feasible for us to distribute the source of everything that
npm downloads and includes when we are building. It thus also doesn't seem
feasible for us to build and ship the compiled (possibly minified) front-end
code. Instead, we have to make it as smooth as possible for our users to
get up and running.

It doesn't seem feasible for us to ship or install npm. We must assume it is
available. That requirement must be documented clearly, and we must recommend
how to install npm for the most common platforms.

We could perhaps just document what manual steps to run. Kallithea doesn't
work out of the box anyway - it has to be configured and initialized. Extra
steps might not be a big problem.

Another approach is to call out to npm while pip is installing Kallithea and
download the requirements and build the files. It can be done by customizing
setuptools commands in setup.py. But: Python packaging is fragile. Even
though we only support pip, it really isn't built for things like this.
Custom output is muted and buffered and only shown if running with -v or the
command fails. And pip and setup.py can be used to build and install in so
many ways that we probably can't make it work reliably with all ways of
installing Kallithea.

The approach implemented by this commit is to add a custom cli command
'front-end-build' to run the required commands. This single user-facing
command can internally run various steps as needed. The only current
requirement is the presence of npm and an internet connection.

For now, this will just create/update style.css ... but currently probably
without any actual changes. The files created by npm (and the node_modules
directory) must *not* be a part of the release package made with 'setup.py
sdist'.

(Commit message is mostly written by Mads Kiilerich)