Master

See merge request !36

- create a new Contributor model, which is just a proxy class for User
- finalize first version of the contributor page
- add contributors to sitemap
- bonus: improve sitemaps to generate only url and lastmod attributes
  (the only ones used by search engines) and use last validated contribution
  for lastmod

TODO: contributor detail page

- remove debug prints
- improve error handling

- use TransactionTestCase and reset_sequences when relevant
- a few minor style and docs improvements

- fix a few problems due to Django upgrades
  - using an expression to sort a list view: F('-tanimoto')->F('-tanimoto')
  - all calls to translation calls ugettext*** replaced with gettext**
  - moved admin site config to a separate module
- forgotten migration added
- dev environment now uses docker-compose
- delete ansible deployment code now obsolete