raise exception when requesting to browse an origin directory/content/history but there is no existing swh visit

ensure display of textual content not encoded in utf-8 in browse content views + add encoding to content metadata

wrap swh object metadata in HTML pre tag for a better display of textual content in browse web application

closes T828

update documentation: port remaining md files to rst and add swh-web modules description in developers doc

closes T800

enable to specify ISO date string as parameter of some endpoints of the swh web browse application (as in the web api)

closes T810

first pass on improving and completing the sphinx documentation for swh-web: reorganize rst files, start documenting the swh web api urls, add example links for swh web browse, ...

this is far to be complete and shiny at that point but it's a good start

  - move django settings for tests in new swh.web.settings.tests module
  - set DJANGO_SETTINGS_MODULE environment variable value to that module in debian/rules

closes T806

note: this was enabled only for swh-web, but it really needs to also be in
swh-docs, because we pull there docs from individual modules

closes T786