dev_4_4: Allow web apps to define their own settings

This PR adds functionality to allow web apps installed through `ADDITIONAL_APPS` to define their own `CUSTOM_SETTINGS_MAPPINGS`, just like in `omeroweb/settings.py`.

The defaults for those settings can then be overridden with the standard `omero config set` mechanism.

This also adds logging for those settings.

--rebased-from #1788