Investigate refactoring to include use case covered by bc-validate

[bdolor]

Description

The similarity between bccampus/bc-validate and bccampus/validate-by-domain justifies the effort to investigate incorporating the use case for bc-validate. Maintaining two plugins that do similar things doubles the effort.

Expected behaviour

The scope of validate-by-domain should be expanded to incorporate more and broader use cases.

Actual behaviour

The scope is currently limited to one particular use case with very specific dependencies (ie buddypress, specific sign-up fields)