Default to the email the invite was sent to

Driving team member invites is a key focus area for Growth. There are a lot of opportunities to increase the invite acceptance rate.

Relates to Q4 KR: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/5396

Current state

When an invite is sent to a non-existing GitLab user, the invited user must first 'create an account'. Currently the user gets an email that invites them to join the project

After clicking 'Join now' the user is directed to the registration flow.

On this step, the user is able to 'change' the email address associated with the invite. This could lead to unintended errors and access issues.

MVC Solution

We do not want to let a user 'change' the email address associated with this invite link.

• Pre-fill the user email and do not allow a user to change it
• Only do this for this invite flow
• Error state see #429936 (comment 1660775254)