if user permissions have been added externally via 'trac-admin' then AccountManager will not allow you to create that user

[anonymous]

if user permissions have been added externally via 'trac-admin projpath permission user add perm' then that user exists already even though they cannot login and AccountManager does not allow you to create the user (gives a User Already Exists error).

I found that if I used WebAdmin->Permissions to delete the permissions for that user, then AccountManager allowed me to add the user

[Matt Good]

In order to address #155, creating users that had existing permissions was disallowed. I guess this behavior should be kept for users registering their own accounts, but this test can be skipped when an administrator is adding an account.

[Matt Good]

(In [1127]) skip permission check when admin creates a new account (fixes #601)