Respect subject prefix configuration for Asana sync and include line count

Summary:
Ref T2852.

  - Respect the existing setting for `"[Differential]"`.
  - Show `[Request, X lines]` to make this more similar to the email.

Test Plan:
Sync'd to asana and got a task with the right subject:

{F49950}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6439