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 signing with OpenID
optional.

Fix bug where full name field is " " by default.
OpenID Plugin: 

Rename config setting
Add helper function to check config setting
Make full name concatinater less insane
OpenID plugin: Missed a config check/function substitution.
Merge pull request #2279 from vanilla/feature/openid-optional-signin

OpenID Plugin: Make signing in with OpenID optional.