Create group invite links

[Repumba]

Your checklist for this pull request

• [X] I've read the contributing guideline.
• [x] I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• [x] I've added automated tests for my change (if applicable, optional)
• [ ] I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

You cannot invite users to your group

What is the new behaviour?

You can invite users to group After inviting, user receives email with link Invited user can click the link and join your group You have to be group admin to invite users Links expire after 7 days by default, you can configure it

Test plan

I've added automatic tests

Closing issues

closes #824