optionally serve up front-end apps for all-in-one scenarios

[leifj]

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Back in 1.x pyFF had build-in frontend apps for discovery and metadata visualization. These have since been made into separate projects. In order to enable simple "all in one" deployment scenarios this PR attempts to make it possible for pyffd to serve frontend apps from the embedded gunicorn server. This will not be the default but it will make it possible to launch pyffd with the same feature-set as during 1.x

[coveralls]

Coverage decreased (-0.7%) to 71.332% when pulling 75bf399f46f7a8f1de63894e26a559b62ecd2f33 on leifj_feat_bundled_apps into 052c5561657348a85b099516bbbe8af8affab023 on master.