Improve email invitation

[CalamityC]

Rationale / Begründung

In a cross site invitation the invited user's first (in database) site is choosen to build the invite link in the invitation email.

Minimal functionality

Easiest solution could be to use the inviter's site in the link. But that might not be appropriate for all sites.

A better approach could be to list links for all sites the user is registered on and let the user choose on which site they want to see that project.
This list should also contain the names of the sites, not only the link.
The text of the email template should be adjusted accordingly.

Affected

All

Nice-to-have functionality

If users are not yet registered to any site (and are invited via their email address) the inviter's site is used as domain for the link.

If that seems not reasonable maybe a settings entry could name the possible sites that should be listed in this case.

[jochenklar]

Idea for a feature flag:

• Add PROJECT_INVITE_USE_PROJECT_SITE = False in core/settings.py (after PROJECT_INVITE_TIMEOUT).
• Use settings.PROJECT_INVITE_USE_PROJECT_SITE in get_invite_email_project_path to distinguish between the 2 cases.
• Use project.site to obtain the site for the project in the PROJECT_INVITE_USE_PROJECT_SITE = True

(Like all settings, PROJECT_INVITE_USE_PROJECT_SITE can be set (to True) in config/settings/local.py.)