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
c9dc554cb9