User signup verification through a list of pre-approved registrar

[armintalaie]

Currently, the Starport Userbase API supports a few verification checks, e.g., Google account connections. However, a crucial step is to only allow a pre-determined list of emails.

Background:
LP has been using Github to ensure this flow, which wasn't the most secure way. Also harder to keep track of this.

Proposal:
a sub-endpoint for the userbase API to add/remove emails to a newly made table registrar. This endpoint only allows user agents with admin auth (i.e. highest scope)