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.
Closes T2360