Generate ?pmpro_level=X instead of ?level=X

[andrewlimaza]

Describe the bug
If you create a custom field with the name and key of "level", and try to checkout while using the previous ?level=1 query param your checkout fails.

This is automatically generated in the "Your Level" or "Select" button and should be updated to use ?pmpro_level insteadand can support backwards compatibility if needed.

To Reproduce
Steps to reproduce the behavior:

1. Create a User Field (select) named level and make it required.
2. Setup the Advanced Levels Add On and choose a level from the levels page.
3. Try to checkout.
4. See error

[andrewlimaza]

An alternative is to rename the User Field "level" to something else like "my_level"