Refactor enrollment check to lookup groups for funding source

[thekaveman]

This is blocked until cal-itp/littlepay#60 is released.

Right now we lookup all funding sources for a given group, and check if the current user's funding source is in that list.

This change will invert the lookup -- we'll get the groups for the current user's funding source, which is likely a much smaller list to check.

Acceptance Criteria

• [ ] The re-enrollment check happens by looking up groups for the current user's funding source, and checking if the group they are attempting to enroll in is in that list

[thekaveman]

Oops, I opened #2068 forgetting that I had already opened this one. I like the description in that one better, closing this as a dupe.