Batch download originals jpegs

This was originally opened as https://github.com/openmicroscopy/openmicroscopy/pull/2220, but has recently been rebased and further commits added to bring it more into line with Insight.

When single or multiple images are selected:
 - the download menu has "Download" option for downloading the original imported files
 - the download menu has "Save As JPEG" for downloading as JPEG.

Both of these options will directly download the file if only a single file.
If multiple files need to be downloaded, you will get a dialog for zip download (with the option to change name of zip).

We are still missing 'Save As PNG / TIFF' since it is not technically feasible to have a 2-level menu here in web. Could add 'Save As PNG' and 'Save As TIFF' to the menu if needed?
If we state 'xhtml' then do we need `<br/>` also?