[FEATURE] [MER-4132] Reimplement authors invite functionality

[nicocirio]

Still pending:

Fix Invite New Authors link

Already done:

Link to the ticket

As discussed in the main ticket, this PR also introduced the :pending_confirmation and :rejected statuses for and project collaborator (authors_projects table), to allow the author to decide between accepting/rejecting the invitation (in the previous version the author was directly added to the project and the email invitation ended up being an information email)

So, the project's overview page was expanded to allow the author to see the collaborators' possible statuses (rejected, accepted, or pending confirmation).
On the other hand, now the course author workspace won't show the project until the author ends up accepting the invitation (in a future iteration we might want to show a pending confirmation project as "grayed" with a link to the AuthorsInvite liveview)

A non-existing author invitation flow

non_existing_author.mov

An existing author invitation flow (not logged in -> password must be provided)

existing_author_not_signed_in.mov

An existing author invitation flow (logged in with the same account as the invitation)

Since the author is already logged in, the author invitation is confirmed and the author gets redirected to the project as soon as he/she clicks the "Accept" button

existing_author_signed_in.mov

An existing author invitation flow (logged in with a different account than the invitation)

The author sees a warning and will be logged in with the account corresponding to the invitation

existing_author_with_another_account_logged_in.mov

An already rejected invitation

<img width="1434" alt="rejected_invitation" src="https://github.com/user-attac

An already accepted invitation

hments/assets/0ac0ad3c-4f8f-4b48-8b0a-3d95bb96c79b" />

A non-existing invitation

Collaborator invitation's statuses on the Project Overview page