web service - ownCloud Web

[mmattel]

The web service env's have configurations for owncloud web.
owncloud web uses environment variables from the web service and/or config.json and themes.json where the envs overwrite any *.json value set if applicable. This needs documentation in the web service documentation

Referencing:
owncloud/docs-webui#48 (Make "Configuring ownCloud Web" a partial)
owncloud/docs-webui#49 (Document theming for ownCloud Web)
(more to come)

@kulmann fyi