Thumbnail by section

This adds support for Z & T to the webgateway/render_thumbnail url, as an alternative to #3278.

E.g. webgateway/render_thumbnail/3455/?z=10

To test, try various z/t combinations.

Also test that you can set the default Z/T by POSTing like:
```
$.ajax({type:"POST", url: "http://localhost:4080/webgateway/saveImgRDef/3455/?z=22"})
```
Refreshing the thumbnail (without specifying z or t) should update to the default z or t set by this POST.

--no-rebase