Big image zoom init 8475

NB: This PR is on top of https://github.com/openmicroscopy/openmicroscopy/pull/979 and only the last 2 commits are specific to this PR.

Web Big Image viewer should initialise zoomed out as much as necessary to fit in the window, or as much as is supported by the server zoom levels. Equivalent of Insight PR: https://github.com/openmicroscopy/openmicroscopy/pull/940
To test:
 - Open a 4k x 4k image and a much Bigger image in the web image viewer.
 - The 4k x 4k image should be partially zoomed out, so that it fits in the viewport.
 - The Bigger image should be fully zoomed out (still won't fit in the viewport).

Also, we now use a thumbnail for the birds-eye view: https://github.com/openmicroscopy/openmicroscopy/pull/967
 - Check the birds_eye view loads OK
 - NB: When you update & apply the rendering settings to the big image, birds eye thumbnail will not update.