[Feature] Add administrator to an event

[HugoGresse]

The event creator or other admin should be able to add other member who will have the same rights as the other on this event.

This feature is ready on Firestore + security (project.members[] is already an array of user id) but need the frontend to be done.

Design: https://www.figma.com/file/EivIdkyPELnGs416ND0C2bWG/Open-feedback?node-id=110%3A1621
TODO:

• Design + email design
• Base admin page listing the admin
• User table with createdAt, updatedAt, displayName, email, photoURL, uid + security rules in order to be able to retrieve it's name, email, photoURL
• When user sign in, we add it to the user table
• Email setup
• Invite system: cloud function + email + UI

[HugoGresse]

Invite process:

1. Current admin invite user by email.
2. in a new 'userInvite' collection, we store the invite:

uniqueId: {
   projectId,
   originUserId: currentUserId,
   destinationUserInfo (could be email or phone),
createdAt,
updatedAt
}

Only allow email invite for the moment.
3. From cloud function, when a new value is put here, we send an email with generated link like "https://openfeedback.io/admin/join?id=XXX

4. a. if the user id already phone in base, add it to the project
5. b. if not found in user, when he is created, add it to the project
6. User arrive on admin root (/admin/), we check the invitedId from the queryParams, get the invite, listen in realtime, if the status = completed, redirect it to the project

Progress:

• store invite in db + security
• send email when newInvite is created from CF
• check if present in db to add the rights directly when invite is created
• when user is created, check is there is pending invite, if so, process the pending invites
• email redirect + modal open to indicate the process
• check if already invited + UI
• add notice for email/pwd login for the email to be verified

[HugoGresse]

Completed!
See https://github.com/HugoGresse/open-feedback/releases/tag/v0.11.0