Discourse Race Condition in Accept Invite

Discourse is an open source discussion platform. Prior to version 3.0.6 of the `stable` branch and version 3.1.0.beta7 of the `beta` and `tests-passed` branches, more users than permitted could be created from invite links. The issue is patched in version 3.0.6 of the `stable` branch and version 3.1.0.beta7 of the `beta` and `tests-passed` branches. As a workaround, use restrict to email address invites.