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 (moved)).
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
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I don't see any downsides to this change, any objections?
cc: @dzaporozhets