Add eager load paths to help prevent dependency load issues with Sidekiq workers

_Originally opened at !3545 by @stanhu._

- - -

Relevant resources:

- https://github.com/mperham/sidekiq/wiki/FAQ#why-doesnt-sidekiq-autoload-my-rails-application-code
- https://github.com/mperham/sidekiq/issues/1281#issuecomment-27129904
- http://blog.arkency.com/2014/11/dont-forget-about-eager-load-when-extending-autoload
- https://github.com/rails/rails/blob/52ce6ece8c8f74064bb64e0a0b1ddd83092718e1/railties/lib/rails/engine.rb#L472-L479
- https://github.com/rails/rails/blob/v4.2.6/railties/lib/rails/paths.rb

Attempts to address #3661, #11896, #12769, #13521, #14131, #14589, #14759, #14825.

See merge request !3724