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?
It seems to be asking for mistakes to have multiple spellings of "centre" in the same file; perhaps this can be made consistent.