browse/snapshot_context: Refactor, add type annotations and tests

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

Closes T2360