Provide default image alt text in more contexts and support custom alt text

Summary:
Ref T13629.

• Allow files to have custom alt text.
• If a file doesn't have alt text, try to generate a plausible default alt text with the information we have.

Test Plan:

• Viewed image files in DocumentEngine diffs, files, {Fxxx} embeds, and lightboxes.
• Saw default alt text in all cases, or custom alt text if provided.
• Set, modified, and removed file alt text. Viewed timeline and feed.
• Pulled alt text with "conduit.search".

Maniphest Tasks: T13629

Differential Revision: https://secure.phabricator.com/D21596