Merge upstream PRs

[Miradorn]

There are MRs upstream that solve existing problems:

• [x] Don't create compile-time dependencies to embedded schemas #86
  • done in https://github.com/remoteoss/polymorphic_embed/pull/8
• [X] Traverse nested PolymorphicEmbed errors #78
  • done in https://github.com/remoteoss/polymorphic_embed/pull/12
• nil on data when calls from ex-machina #76
• WIP: ex_machina support #26

Also look at these issues:

• Polymorphic embeds are not kept as changesets but are turned into structs - as opposed to what happens with ordinary embeds · Issue #74 · mathieuprog/polymorphic_embed

[tmjoen]

There has been a lot of great work in this fork as well:

https://github.com/steffende/polymorphic_embed