Some thumbnails have not been generated from media uploaded through form

[mrengy]

Is this an ongoing issue, or something that only took place once? If it's an ongoing issue, fix it long-term.

If we don't expect it to occur any more, perhaps just regenerate thumbnails for old media.

We can tell which thumbnails have not been properly generated - they appear as rectangles rather than squares in Media Library admin screens.

This likely impacts other image sizes that should have been generated as well.

Related to #37

[mrengy]

In progress using WP CLI to regenerate thumbnails. Getting errors about exceeding max execution time on stage. Tried to increase max execution time but was unsuccessful. Asking Dreamhost support for help.