As a user I want to invite members by email to my group

Problem/Motivation

It is currently only possible to invite people as a Group manager, but we need to make the invite function accessible to LUs.

Remaining tasks

- The email contains a button pointing to the group about page where the user can accept the invite
-- This is only for groups with visibility public or community
-- For secret groups, the link points to /my-invites

- Once the user does not yet have an account or is not logged in, after getting access via EU Login they are pointed to either the group about page or /my-invites page (depending on the criterium above)

- If Group Manager allowed invitation by members for the group, members can invite users to the group via the normal members' screen

- Member should be able to copy link to the group and share it:
-- it should be a normal browser link, not a link to the invitation
-- follow the logic implemented for YPO post sharing in order to keep things consistent

- Member should be able to send an email:
-- link to the group should be included
-- pre-defined group invitation text should be used as a message

- Implement in social_group_invite (check with Nucleus)

- Add automated tests