Web-based interface for browsing the archive (1st public version)

One of the main feature that the SWH web user interface will have to offer is the easy browsing of a source code tree (like any software forge web interface). As a first task for the beginning of the development of the swh web browse ui, let's implement it.

The associated sub-tasks are the following:

• add basic views for browsing directories and content objects from their sha1_git checksums
• add views for browsing those objects from a specific origin and its visit by swh.
• add view for browsing the history log of a specific revision
• add a simple interface to search software origins to browse based on their urls

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Migrated from T759 (view on Phabricator)