Ldap authentication: get all users even before they login the first time

**Is your feature request related to a problem? Please describe.**
I've just setup weblate to use ldap authentication. I configured it in the way that only users, which belong to the group "translation" can log into weblate. But now if I want to add a new user, and assign them to a certain project, give permissions, etc. I need to do:
- Add him to the group translation
- Write him an e-mail with what he has to do, but tell him he can't do anything, before he logged in the first, time. So he should write me back once he logged in once.
- wait until the user logged in and tells me so
- go into weblate and configure his permisions, assign him to projects, etc.
- write the user, that now he can start working

**Describe the solution you'd like**
That weblate gets all the users with the right group directly from the ldap directory (it could be a button in the admin interface). So I can configure users in only one single step.