misc: Add iframe view for contents and directories
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.
Below is an example of how to integrate that view in an iframe:
<iframe style="width: 100%; height: 500px; border: 1px solid rgba(0, 0, 0, 0.125);" src="https://archive.softwareheritage.org/embed/(swhid)/"></iframe>
And this is how it looks:
Related to #3489 (closed)
Migrated from D6186 (view on Phabricator)