Replace Devise Async with Devise's implementation of async emails

Devise can integrate with ActiveJob with a bit of configuration, no external gem dependency needed. And it works with Devise 3.x!

Devise Async hasn't been updated since July 2015 and doesn't support the newly-released Devise 4 which will be necessary for Rails 5 (#14286 (closed)).

This is the commit that adds devise-async and it seems that it mostly hasn't been updated since, seeing as we're still on ~> 0.9.0.

I don't see any downsides to this change, any objections?

cc: @dzaporozhets