Alt text support

[skullbunnygalaxy]

Research alt text implementation for accessibility, including adding alt text to image uploads and editing that data later.

Part of the difficulty is figuring out how best to store this data, ie as part of an image upload or as a separate item, which would require the user to add this information after an image upload.

[zkat]

Note: Repurposing this issue as a general "alt text for uploads" support thing. It'll be extra important when #232 lands.