Display image alt text

[micahmo]

Pull Request Description

This PR adds support for showing alt text in the image viewer.

Notes:

• This change supports image posts, as well as images in comments which use the markdown alt text feature.
• Long text is handled with an expander, similar to taglines.
• The alt text will be hidden when long-pressing the image, which hides all other content on the screen besides the image.
• This PR does not add the ability to set the alt text creating an image post. That will require a database change (to save it with drafts), so it should come after Support custom thumbnails in posts #1495.
• I verified that the white text still looks good against a solid white picture due to the shadow.

Issue Being Fixed

Issue Number: #1447 and #1461

Screenshots / Recordings

qemu-system-x86_64_gOcyq5564F.mp4

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

Just a small minor comment, otherwise LGTM!

[hjiangsu]

Nitpick: use l10n conventions here!

[micahmo]

Done!