Limits number of SSO / Login ID a user account could have

Currently, say if Email Login ID is enabled, in Portal, admin seems can add unlimited number of email address; While in Setting, users can only have 1 email address (and display latest one only seems)

We should:
- Make it a configuration, default 1 only
- Sync Portal and Setting behaviour under different settings.

Start by tracing how Portal and Setting add and display login IDs, then identify where account-level limits and configuration are defined. Confirm the intended default of one and the behavior for higher limits before changing either path. Done means both interfaces enforce and present the same configured limit.

Written by the indexing model from the issue text.