Add a 15-second timeout to external service calls to fill Doorkeeper link tags

Summary:
Depends on D20528. Ref T13291. Ref T13285. Currently, we don't put a timeout on external service calls when enriching URIs for external Asana/JIRA tasks.

Add a 15-second timeout so we'll do something reasonable-ish in the face of a downed service provider. Later, I plan to healthcheck Asana/JIRA providers in a generic way (see T13287) so we can stop making calls if they time out / fail too frequently.

Test Plan:

• Linked to JIRA and Asana tasks in comments.
• Set timeout to 0.0001 seconds, saw requests time out.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13291, T13285

Differential Revision: https://secure.phabricator.com/D20530