This makes models file more readable

It is easy to do and allows user to use any URL to access repos.

Simply calling string is not really way to call such named method.

We use ttkit for constructing the file, pretty much same approach could
be used for other formats as well, but I'm not sure what other would be
useful to support.

Fixes issue #210

Currently translated at 85.3% (656 of 769)

Bitbucket hook

https://github.com/nijel/weblate/pull/208#discussion-diff-3009481

Less code duplication and the _hook_helper functions returning an
HttpResponseBadRequest() is not handled by git_service_hook()
either way.