Reorganize and refactor snapshot browsing related code

Currently, code to retrieve snapshot context related data and prepare HTML display is spread in multiple source files:

• there is a get_snapshot_context function in swh.browse.utils module
• the rest of the code is located in the swh.web.browse.views.utils.snapshot_context module

This code organization is not great and must be merged in a single utility module swh.web.browse.snapshot_context.

Also that code must be refactored and improved (notably by renaming some variables, removing code duplication, adding type annotations and missing tests).

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Migrated from T2360 (view on Phabricator)