Add support for ?show=fileset-123

[will-moore]

Often it's useful to find Images etc by Fileset, e.g. when an import completes with...

...
Other imported objects:
Fileset:6314846

==> Summary
444 files uploaded, 1 fileset created, 1 image imported, 0 errors in 0:13:35.110

This PR adds support for webclient/?show=fileset-6314846. Since we can't select a Fileset in the webclient, we simply pick a Image from that Fileset, similar to support for ?show=roi-123.

[knabar]

@will-moore Works fine for me, but it does seem to pick the last (or a random?) image in the fileset; would it make sense to always select the first image (presumably lowest ID)?

[will-moore]

Thanks @knabar - fixed now to pick the "first" image (lowest ID).

[pwalczysko]

built the omero-web locally as our merge-ci is not available.
Indeed, when a second image in the fileset is selected, than a url like /myserlver/weblcient/?show=fileset-79 shows the first image in that fileset. Lgtm. Thanks @knabar for going ahead, just adding this for completeness sake.