Enhancement: Embed mode

We've introduced a so called "Embed Mode" that allows Web to be consumed by another application in a stripped down version. This mode is supposed to be used in the context of selecting or sharing resources.

Please see our documentation for more information and a guide on how to set it up.

https://github.com/owncloud/web/issues/9768
https://github.com/owncloud/web/pull/9841
https://github.com/owncloud/web/pull/9853
https://github.com/owncloud/web/pull/9863
https://github.com/owncloud/web/pull/9981
https://github.com/owncloud/web/pull/10113
https://github.com/owncloud/web/pull/10071
https://github.com/owncloud/web/pull/10076
https://github.com/owncloud/web/pull/10082
