Allow user account merging via email

[julianlam]

NodeBB no longer adds users to email:uid unless email is confirmed. Similarly, session-sharing when creating new users will also not have the email confirmed.

A new feature flag (or flags) should be added to allow session sharing to:

1. Allow user account merging via email (this used to be automatically true, it will now be default disabled, hence it is a breaking change)
2. Automatically confirm user emails on user creation