Default to site default role

[andrewlimaza]

• ENHANCEMENT: Added the ability to create the pmpro_role_<<level_id>> when we detect the level doesn't have this role created and not only on initial level creation.
• ENHANCEMENT: If no role is selected for a level, it automatically selects the sites default user role and doesn't create the custom role anymore.

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[andrewlimaza]

Resolves: #31

[andrewlimaza]

This also resolves some issues with PMPro 3.0+