Sign and Register with weblate for self-hosted weblate instances

**Is your feature request related to a problem? If so, please describe.**
It's always annoying when has to register and sign in each time with credentials for self-hosted instances otherwise we need keep login for github or google etc non-relation third-party services for make that easy (even it, different instances use different third-party services)

**Describe the solution you'd like**
Introduce new authentication option as "Sign in with weblate", " Create an account using weblate" and put it on the top of auth options list as common. provide it as built-in **hardcoded** config and enabled by default (with simple option to set hosted_weblate_auth=false if necessary to disable)

**Why it is perfect**
Nearly all volunteer translators (currently upto 30,000) join to hosted weblate projects to contribute then i believe that's more appropriate and avoid feel upset when use weblate's on-premise instances