feat: add endpoint to invite users

[ojeytonwilliams]

Builds on top of #342 (please don't merge until that's in!)

The workflow this allows is

1. create a user via POST /api/auth/local/register
2. invite that user via PUT /api/auth/invitation/:id

This creates a user that can only be logged into via Auth0 and sends them an email letting them know they can log in.

To be defensive we can create the user blocked and with long random password, but the main protection is that will disable the email provider in production. It prevents email/password logins, but not registrations, so does not interfere with this workflow.

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Why?

This simplifies the invitation process and decouples it from registration. We no longer need to maintain a separate content-type whose purpose was to prevent self-registration. Instead we can allow registration only by Administrators.

It also simplifies roles. When I was trying to understand how user registration worked, I was confused by what invited-user.role did and how it differed from user.role. It's not hard to understand, but it was extra complexity.

[sidemt]

@ojeytonwilliams Thank you for this proposal.
I haven't checked the new flow thorougly, but a few things I wanted to note:

• In our use case, editors are most likely to be someone from the staff team, so we wanted to simplify the roles into 2 types, Editor and Contributor.

• Users (both editors and contributors) will only use the Next.js app and will not use the Strapi admin panel. So we will still need the /users page (linked from "Staff" in the menu, the file is src/pages/users/index.js) and the user invitation functionality in there.

We can discuss this in the upcoming platform team meeting.

[ojeytonwilliams]

In our use case, editors are most likely to be someone from the staff team, so we wanted to simplify the roles into 2 types, Editor and Contributor.

Thanks Yoko, I'll adjust #350, but this PR should be okay.

Users (both editors and contributors) will only use the Next.js app and will not use the Strapi admin panel. So we will still need the /users page (linked from "Staff" in the menu, the file is src/pages/users/index.js) and the user invitation functionality in there.

Yep, understood. I'll put it back in before I'm done updating the invitation flow.

[Nirajn2311]

LGTM