browse/snapshot_context: Put processed snapshot sizes in cache
browse/snapshot_context: Put processed snapshot sizes in cache

Put processed snapshot sizes in cache to avoid sending the same set of
database queries each time a page is loaded when browsing the archive
in the context of a specific snapshot.

Closes T2734

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui