Custom web login 11961 (rebased onto develop)



This is the same as gh-774 but rebased onto develop.

----

This documents changes in https://github.com/openmicroscopy/openmicroscopy/pull/2461

My only question is exactly what is required when replacing the png after web deployment.
When I tried this with my local nginx deployment, it seemed to cache the original png for a while (even after browser refresh).