Add a new Django view that can be embedded in an iframe to display
contents and directories archived by Software Heritage in a minimalist
Web UI.

The URL targetting that view is /embed/(swhid)/.

The view enables to navigate in a source tree and to display SWHIDs
of currently browsed objects. It also has a fixed header with a link
to softwareheritage.org and a link to browse current object on the
main archive website. Code fragment highlighting for content objects
is also supported if the input SWHID has a lines qualifier.

Related to T3489