Add OMERO.web viewer view configurability

[chris-allan]

Add OMERO.web viewer view configurability which allows for the complete override or installation of a redirect view for the existing full viewer. This is particularly useful where an OMERO.web plugin is being used in production that replaces or extends the current full viewer, during development or during proof of concept work on new full viewer technology.

Out of the box there should be no affect on existing functionality. The additional configuration option is omeroweb.web.viewer.view and this accepts a fully qualified view function name which defaults to omeroweb.webclient.views.image_viewer. It can be tested at a very basic level by running the following:

    bin/omero config set omero.web.viewer.view 'omeroweb.webclient.views.list_scripts'

This will display the current list of scripts in the full viewer window, as JSON, when an image thumbnail is doubled clicked on or the "Full Viewer" button in the right hand panel is clicked.

If a plugin wants to perform a conditional redirect, maintaining existing full viewer functionality for the majority of images, adding the following to its urls.py is likely desired:

...
    url(
        r'^vanilla-viewer/(?P<iid>\d+)/$', webclient_views.image_viewer,
        name='myviewer-vanilla-viewer'
    ),
...

A view similar to the following, which would be the target of omero.web.viewer.view, is then possible:

...
@login_required()
def viewer_redirect(request, iid, conn=None, **kwargs):
    """
    Use regular viewer, except when image name contains 'test'
    """
    image = conn.getObject('Image', iid)
    if image is not None:
        try:
            if 'test' in image.name:
                return HttpResponseRedirect(reverse('myviewer-viewer', args=(iid,)))
        except:
            pass
    return HttpResponseRedirect(reverse('myviewer-vanilla-viewer', args=(iid,)))
...

/cc @jburel, @joshmoore

[will-moore]

This works fine.
It would certainly be useful to have the instructions and code samples above included in our documentation, so that users know how to use this config option, since I think it would often be the case that you'd want to retain the original viewer for some images.
Good to merge.

[jburel]

@will-moore: since you have been updating the doc for the web plugins support, could you take care of adding a section for the new extension option?

[jburel]

@will-moore is taking care of the documentation update.
Merging thanks @chris-allan

[will-moore]

Docs PR at ome/omero-documentation#1406