Make mail_room idle_timeout option configurable.

What does this MR do?

Allows configuration of idle_timeout for incoming email.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We had a hardcoded value of 60. For GitLab.com we need this to be configurable.

Screenshots (if relevant)

Not relevant.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1087