On the road to T2342, second part of T2360.
Slightly refactor and improve the swh.web.browse.snapshot_context module:
- merge _process_snapshot_request function into get_snapshot_context one
- rename some dict keys and add snapshot / origin related type annotations
- cleanup and simplify get_snapshot_context implementation
- update some docstrings
- add new exhaustive tests for get_snapshot_context function
That diff is quite large as I performed some keys renaming in dictionaries spanning
numerous files. I have also added mising tests.