Contribution pages with memberships don't validate correctly when using "quick config" price fields

In CRM_Contribute_Form_Contribution_Main::formRule, there are multiple references to $self->_useForMember. $self->_useForMember is only TRUE if the price set is a "quick config" price set (and is explicitly used as such in the template). However, during formRule validation, it's used as a proxy for "this contribution page uses memberships". @mattwire correctly identified the problem on the confirmation class in PR 15094.

As a result, several membership-specific validations are bypassed.

I attempted to move his methods to the shared ContributionBase class and make them protected, but I ran into deeper validation issues. I can't justify further work on this, but wanted to document for future folks who bang their heads against this.