Only build locally-served interface

Remote connections are now handled by https://gitlab.com/openflexure/openflexure-connect/

To simplify the code, we are switching this repo to only build the GUI that is served locally from the root of the microscope server.

Main effects of this:

• CI no longer builds this repo
  • OpenFlexure Microscope Server instead pulls this repo in as a submodule, which gets built as part of the server release
• No electron build-target
• API URL is no longer variable
  • It can be changed for debugging purposes, but by default the app assumes the API is on the same host as itself
• No Connect tab

@rwb27 Are you happy for this to go ahead? If we need to patch v2.1, we have a hotfix branch, so this shouldn't affect patch releases.