Generate welcome link in user profile

Often, an administrator wants to assist a user who forgot their password, or has just been imported in. Add a simple button in the user details, where global administrators can (if welcome is enabled) generate a welcome link for users.
Merge pull request #342 from phihag/welcome-link-in-user-profile

Generate welcome link in user profile
Merge remote-tracking branch 'liqd/develop' into welcome_back

Conflicts:
	src/adhocracy/controllers/user.py
	src/adhocracy/lib/auth/welcome.py