Reset Rendering-defs and recreate Thumbnails

[will-moore]

See discussion at
https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=8728

It would be useful if the CLI render plugin could reset rendering defs (to the imported settings) and regenerate thumbnails (probably as 2 separate commands).

This functionality was tested in this script (improvements suggested in discussion above)
https://gist.github.com/will-moore/7a37a2c48bbfc5a247cb54c2d876c79c

It might be sufficient to only support thumbnail generation as this will also create rendering defs (if they've been deleted as in workflow above).