Support image galleries in featured data articles

[clnsmth]

Featured data authors have expressed interest in sharing a collection of images to help communicate data collection processes and other interesting aspects of a project. These interests align with our goals for the featured data series.

Currently, adding several images to a featured data article results in an image heavy page where the majority of the vertical scroll space is occupied by images and very little article text, which is unconventional to say the least. Typically, webpages use galleries to share an image collection, where the gallery is composed of clickable thumbnails that expand into full sized images.

One potential solution here is to implement a gallery with an HTML block, embedded in the content-x markdown, and referencing CSS to enable a flexible layout with consistently sized thumbnails. This latter feature is beneficial to article curators who would otherwise have to create a separate set of thumbnail images.

For example:

• An article containing a collection of images in the current implementation
• An article with the proposed gallery implementation
• The corresponding article in content-x as markdown
• The supporting CSS

A couple shortcomings of this implementation to be aware of:

• It is not possible to iterate through all images from the enlarged image window. The user must back out of the enlarged image to the main gallery and click on the next image to enlarge it. A more streamlined user experience would allow direct iteration from the enlarged viewport (see here for an example).
• Individual image captions must be supplied in a single block of text below the image gallery.

[clnsmth]

Relatedly, a set of contributing guidelines have been staged at https://github.com/PASTAplus/content-x/commit/1d33882cc80a7167a1285d7c6ebaeb2cf6d907e4 and are ready to be merged if the proposed feature here is excepted, as is.

[clnsmth]

During review, it was decided that some additional "bells and whistles" would be nice to include as time permits, specifically:

1. Support navigation between fully enlarged images of a gallery in a manner consistent with a modal image gallery (lightbox).
2. Support image specific captions. Note, the lightbox option above, supports this through use of the alt attribute of the HTML img tag.

[clnsmth]

The related set of contributing guidelines has been added via https://github.com/PASTAplus/content-x/pull/43.