This commit adds the following improvements:
  - landing page for browse in now the origin search interface
  - use a navbar instead of tabs in browse in order to get
    more vertical space for content display
  - remove the default browsing of torvalds/linux
  - some rework on the search interface

Closes T924

Close: T884

Your branch is up-to-date with 'origin/master'. to the view documentation

Turns out that the parameters for the underlying PostgreSQL
requests were not adequate for an optimized search.
Origins search is now pretty fast.

Closes T878

Use intrinsic info to reference SWH origins and visits in
the browse URI scheme (see updated doc):

  - origins are now referenced by their type and their url

  - visits are now referenced by their date

Also add a new query parameter 'visit_id' to disambiguate
visits with the same date.

Closes T877

Enable to add an origin context through query parameters when browsing a revision.
This should only be used internally by the browse application or for
referencing a SWH deposit as no checks are performed to ensure the
origin and the revision are tight.

Closes T875

Add client-side rendering of README files using the showdown
javascript library (https://github.com/showdownjs/showdown)

When browsing an origin, the branch named 'HEAD' or 'master' is set as
the default one to retrieve. But some origins do not have such
branches (e.g. Debian source packages), so in that case, retrieve
the first branch found.

Related T871

Closes T848

Related T848