Improve differential.createinline

Summary:

• Share code between createinline and getcomments
• Make them both extend the base class.
• Sync up the parameters (this is more or less nonbreaking since the call is ~6 hours old).

Test Plan: Created and fetched inlines via the API.

Reviewers: alanh, vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1500

Differential Revision: https://secure.phabricator.com/D2988