Remove or factor out hiccup-elements

The core logic (`openid.clj`) contains several hiccup-templates. It is nice to provide this as a starting point for potential users but not strictly/technically necessary here.

Two options:

1. Remove the templates from the source-code and instead provide them as part of some documentation that shows how to use the library when you need this specific auth-flow
2. Keep the templates in the code but move them to their own utility-namespaces so they don' clutter the core logic.