OpenID Plugin: Make signing in with OpenID optional.

Since the OpenID plugin is requisite for other plugins,
make the ability for users to be able to sign in with OpenID
optional.

Fix bug where full name field is " " by default.
Don't do this. Just do the following:

1. Read the setting like so: `C('Plugins.OpenID.AllowSignIn', true)`.
2. (My Choice) Flip the config setting meaning to: `Plugins.OpenID.DisableSignIn`.