Truncating of resource gallery cards splits strings mid-word

[jukent]

The current string splitting method in the _extensions/yaml_gallery_generator.py cuts strings off at a certain length (with a recent fix in #147 to not break any html). This is done when the description for a resource is longer than a certain length, and a "...more" modal is added.

Ideally this string-splitting would be a little more sophisticated and not split strings mid-word, but either before or after the closest word to our desired length.

[kmpaul]

This is done with the merging of #147.