Unable to login if user is masqueraded by orphan alias

Steps to reproduce

1. Create a user 'alice' in Crowd, give her access to an application (e.g., Crowd itself)
2. Define an alias, e.g., 'beth' -> 'alice'
3. In a different browser, log in as 'beth', it should work fine.
4. Back to Crowd, delete user 'alice'.
5. Create a new user in Crowd named 'beth', give her access to the application.
6. In a different browser, log in as 'beth'.

Expected outcome

'beth' should be able to log in.

Actual outcome

'beth' cannot log in. This is because the alias 'beth' -> 'alice' is still stored in the database (CWD-2179), and it's masquerading the actual 'beth'.

Note that the problem can be reproduced as well if in step 4 'alice' is renamed, e.g., to 'maria'.