Webtest removal

--rebased-from #3159

This PR removes the /webtest app from omeroweb.

The app is only used for examples and prototypes. It has been moved to it's own repo: https://github.com/openmicroscopy/webtest/

To test, check that the webtest app can be added to OMERO.web, following instructions on the webtest repo.
Also add the center and right plugins as described.

Go to /webtest and try some of the links / pages.
Check that center panel split-view plugin works with datasets and images and the ROI right plugin shows ROIs on images.