browse: Merge snapshot context related functions in dedicated module
First part of #2360 (closed) and not really interesting to review.
The diff simply moves existing code to a new module swh.web.browse.snapshot_context
.
Some import reorganization and URL regexps polishing in swh.web.browse.views.origin
have also been performed.
Migrated from D3021 (view on Phabricator)