Added resend action to admin (Closes #263)

[demestav]

Closes #263

Added a resend action to the admin that (as the name suggests) sends again the email.

Notes:

1. How to handle the po files?
2. This just resends the email, the key of the invitation is the same
3. Since the sent datetime is updated, if expiration is activated in the project, it means that expiration period is also reset. i.e. the user will have x days to activate by the time of resend, if expiry is set to x days.
4. pre-commit last step was failing on my system pycln. Not sure why, but I don't thing it is related to the changes I made.