Allow users to provide alt text for image embeds

Summary:
Ref T4843.

• Add an alt attribute so users can provide alternate text for {Fnnn}.
• Add an alt attribute to image macros.

Test Plan: Embedded an image with alt and a macro, inspected HTML source to verify the alt attribute was present.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4843

Differential Revision: https://secure.phabricator.com/D8925