alt-text transcription field type/metadata field type

[benwbrum]

Some libraries are interested in using FromThePage to create alt-text and other WCAG material. To enable this, we need to create a new type of transcription_field, "alt text", which looks like a regular textarea field, except that it has UI indicators to warn users when they are exceeding recommended length of alt-text contents.

So the UI should let the user type up to N (likely 255) characters, but should start warning them when they exceed 125, then more at 140, then again at 180. (I'm thinking of shading the text yellow, orange, red). We should also have a JS widget under the field showing the number of characters they have typed.