PBC breaks Membership Checkout Form Block functionality

[kimwhite]

Describe the bug
When Pay by Check is active on a site, the Membership Checkout Form Block does not show the set level but defaults to 1, like it has no level.

To Reproduce
Steps to reproduce the behavior:

1. Activate Pay by Check
2. Add the Checkout Form Block to a page.
3. Set it for a level other then the first.
4. When page is viewed reverts to default.

Screenshots
https://capture.dropbox.com/PTBxaYqQ2dKtJ4Sj

[dparker1005]

This may be resolved by #128