Added imjoy support

We now have an imjoy tab and the ability to load plugins. We're working on an API that allows the OFM to be used with the same commands you'd use for MicroManager.

TODO:

• Add an ImJoy tab and load some plugins
• Add a menu to the gallery to open images with ImJoy
• Expose core microscope functions using uManager API
  • Quick snapshot that pulls a JPEG from the MJPEG stream
  • snapImage()
  • getImage()
  • setPosition() (I understand that this defaults to the Z position and is 1D)
  • getPosition() (I understand that this defaults to the Z position and is 1D)
  • setXYPosition()
  • getXYPosition()
  • setProperty()
  • getProperty()
  • fullFocus() (I believe this is a single-shot autofocus)
  • setExposure()
  • We might want to add a few more API functions that effectively add parameters to the above functions.
• Add the ability to load ImageJ as a plugin
• Figure out how to make it work offline
• Add a switch to enable/disable ImJoy support (so we can remove it for medical version)
• Add a way to add tabs to the UI (so long as ImJoy gets a div with an ID it can populate it with the window)
• Figure out a way to get stacks/folders into ImageJ

Nice-to-have:

• ImJoy plugins as their own tabs (rather than within the ImJoy tab)?
• Figure out a way to call the microscope via RPC from a PWA rather than serving locally (we think this may be impossible...)

This MR closes #157 (closed).