Handle many channels

[will-moore]

The key limitations when handling images with multiple channels are:

• Long rendering request string. E.g. Images with more than 50 channels in the render quest string can result in a length over 4k characters which is typically rejected by nginx. Solutions we have used elsewhere:
  • Only include active channels in the request string.
  • Limit the number of active channels allowed. E.g. Support max_active_channels omero-iviewer#500
• UI layout: The long channel list pushes the bottom of the panel (Crop controls) out of view:

This could be improved with a scrollable panel.