Afform - include Generic.html by default

I had a go at adding an afform in an extension today for a simple entity in the extension. I had to do 2 things to expose it

1. Add this file https://github.com/eileenmcnaughton/deduper/commit/2ca0de29f66401584b966dcf42cd299b98af09a7

2. hack a file similar to this one into core https://github.com/civicrm/civicrm-core/compare/master...eileenmcnaughton:grantaff?expand=1#diff-e15ef3dc8f2633f790b8b2b34384aba83b9b5114e01a8f851e9e059273ae42d1

I think 2 could be addressed by fixing this line

https://github.com/civicrm/civicrm-core/blob/a191be25440e213fc29679584d2b62f5076369c2/ext/afform/admin/ang/afGuiEditor/afGuiEntity.html#L59 to include the specified file if exists else Generic.html

Note that simply downloading & enabling the master version of deduper if enough to get to the point where you can attempt to create the afform for ContactNamePair. It DOES work but the options section doesn't render

As a secondary issue, I couldn't figure out how to construct an edit url for it