Issue #170

This requires careful selection of auth providers, but it is what most
people want in Weblate.

Issue #170

Right now only basic login works and is not really integrated into rest
of Weblate.

Issue #170

Admin can now customize who can use machine translation, it is not
longer only allowed for logged in users.

We check permissions anyway and now it is possible to define permissions
for not logged in users.

Currently translated at 27.9% (264 of 947)

Currently translated at 98.7% (935 of 947)

This allows to control privileges for anonymous users.

Currently translated at 100.0% (947 of 947)

Currently translated at 100.0% (947 of 947)

Currently translated at 100.0% (947 of 947)

Currently translated at 99.8% (945 of 947)

Currently translated at 99.6% (943 of 947)