Add request.context to parameters.yml file.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | ---
| Refs tickets  | http://symfony.com/doc/current/cookbook/console/sending_emails.html#configuring-the-request-context-globally
| License       | MIT
| Doc PR        | ---

(I just found this branch of mine which I apparently have never made a PR for)