Add invitations

Includes backend and frontend as separate commits.

We still need to make the generic frontend invitation site that you are redirected to after following the invitation link to the invite specific site in the frontend that does the call to the backend to save the id in the session.

approved this merge request

added 26 commits

• be8a6772...4569fc42 - 24 commits from branch master
• 64967056 - Add backend parts of guest registration
• 7735d369 - Add frontend part of invitations

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changed title from Add backend parts of guest registration to Add invitations

changed the description

added 2 commits

• 467c08d3 - Add backend parts of guest registration
• d8939a77 - Add frontend part of invitations

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resolved all threads

added 1 commit

• 850fd3c0 - Add frontend part of invitations

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approved this merge request

merged

mentioned in commit ad4bf7cd