implement caption and meta-information for images

[tammoterhark]

title, attribution; store in title and longdesc attributes; display as figure with figcaption elements

autofill attribution from exif information

step 1) update WYMeditor image dialog with "attribution"
step 2) save attribution as longdesc attribute at the image
step 3) create and add figcaption element from longdesc attribute
step 4) set "cc: author (date)" as placeholder for "attribution"
step 5) read exif data to autofill "attribution"

[matthijskooijman]

@laurensmartina implemented this in #344, but we discovered that the longdesc attribute is supposed to contain a URL pointing to a description of the image (similar to the alt attribute, but longer). See https://www.w3.org/TR/html-longdesc/#UCnR

So using longdesc here is not quite ideal. There does not seem to be an HTML standard attribute for attribution/copyright/license, so maybe just data-attribution would be better.

As for displaying it, I wonder if displaying the attribution info as a caption makes sense, I'd be more inclined to show the title tag as a caption maybe. Looking around, it seems the <small> tag (meant for the "small print" in HTML5) might be a better fit, see: https://stackoverflow.com/a/21489636/740048

[matthijskooijman]

Additional improvements (as indirectly suggested in the Stadsbron marketing advice) would be to offer more guidance to the users about alt tags, to encourage them to enter a meaningful alt tag (probably also stop autofilling the alt tag with the filename).