This commit moves a couple of helper tasks for developing the frontend
into the gulpfile, and also improves the automatic installation of
dependencies and building of assets.

All that is now required to get a running development server (with built
frontend assets) from a fresh checkout of the repository is:

    make dev

Subsequent runs of `make dev` will ensure the JavaScript and Python
dependencies are up-to-date but will not rebuild the assets unless they
are either explicitly cleaned,

    make clean dev

or the developer runs the gulp `watch` task independently (recommended).

Similarly, from a fresh checkout, it should be possible to run the test
suite simply by running:

    make test

Note that this commit removes `gulp-cli` and `karma-cli` from the
package dependencies -- these packages are not intended to be installed
in a software distribution, although they can both be installed globally
to assist with easier dispatch of gulp/karma tasks if needed:

    npm install -g gulp-cli karma-cli