R1:bfe7cdc5a2af

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…