[keyserver][lib] Block users from creating reserved invite links
Summary:
A user should not be able to create a link which is included in a list of reserved usernames. The copy is similar to a message that is shown when a user tries to create an account with reserved username: /native/account/registration/registration-server-call.js, but I'm not sure if we should mention support email (the user is already registered and can contact us on Comm).
Depends on D8527
Test Plan: Try to create /comm invite link - a message should be displayed.
Reviewers: kamil, inka, ashoat
Reviewed By: inka
Differential Revision: https://phab.comm.dev/D8615