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

implement lines highlighting feature for browse content views (either by clicking on line numbers or using an url fragment like '#Lx' or '#Lx-Ly')

normalize HTML view titles in browse

display visit date when browsing a swh object in an origin context

get rid of that agressive yellow background in the html browsable swh-web api and use the same color as in the browse application

ensure that the autoreload feature of django dev server works when modifying any swh-web source files

closes T491

drops Debian jessie support: remove previously introduced hacks and workarounds in order to support it + update requirements and debian/control

closes T802