Rendering settings: Always show 'SavedBy' Panel (rebased onto dev_5_0)



This is the same as gh-2562 but rebased onto dev_5_0.

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See https://trac.openmicroscopy.org.uk/ome/ticket/12230

To test:
View an image with different users, change and apply the rendering settings for each user, so that this image has several different rendering settings. 
View the image again with different users; check MetadataViewer preview panel and ImageViewer rendering settings panel; make sure that:
- 'Saved by' panel is visible and shows all rendering settings you created before; 
- The owner's rendering settings is marked with blue text for the user name; the rendering setting used for the preview (respectively ImageViewer) is marked with a border.
- Clicking on an item applies it's rendering setting to the preview (MetadataViewer) or the image panel (ImageViewer)
- Clicking on apply/save will overwrite the current users rendering setting with the selected one and update the 'saved by' thumbnails accordingly.