Disable submit and show spinner while editing channels

[will-moore]

Fixes #304.

There were 2 problems with channel editing previously:

• It took a long time (fixed by setChannelNames performance omero-py#331)
• User could hit 'submit' twice since it wasn't disabled when first clicked (fixed by this PR).

To test:

• Edit channel names (either a single image or multiple Images in a Dataset or Plate)
• On clicking 'Save' or 'Confirm', the button will be disabled and show a spinner while saving to server.
• Clicking twice shouldn't submit more than once (can check the Network tab in dev tools to confirm)
• Cancel will reset the form.

[pwalczysko]

Works as expected. Maybe remove the Cancel button, as it does not cancel anything.

[pwalczysko]

Cancel button is hidden, the functionality is working as expected. Ready to merge fmpov.