swh-web: Improve querying / browsing of large snapshots

In order to benefit from a smooth querying / browsing experience of large snapshots:

• plugin snapshot content pagination from swh-storage in the web api and browse
• restrain the default amount of retrieved branches to 1000 and when a snapshot content is larger:
  • add Link header in the api response pointing to the url retrieving the next branches
  • add a warning message at the bottom of the branches/releases list widget
• increase the number of displayed branches/releases in browse dedicated views from 20 to 100
• add/update tests

Closes T1207