Add default queue to Sidekiq config

Contrary to what the Sidekiq documentation[1] says, mail is not put into the
`mailers` queue, but into the `default` queue.

[1] - https://github.com/mperham/sidekiq/wiki/Active-Job

config/sidekiq.yml

@@ -17,6 +17,7 @@
  - [frequency,1]
  - [fixphenotypes,1]
  - [mailers, 1]
+ - [default, 1]
 # - [mendeley_details,1]
 # - [mendeley,1]
 # - [genomegov,1]