Organize HTTP routes better

[ethanjli]

This PR groups the HTTP routes (for static assets, for the GUI, and for the API) for easier reverse-proxying.

TODOs:

• Change the base URL path from / to /imswitch for easier reverse-proxying
• Change the path of the GUI (under the base path) from /imswitch/index.html to /ui/index.html, so that the full URL path is /imswitch/ui/index.html
• Group the paths of the API routes in to /api, so that the full URL paths all begin with /imswitch/api
• Ensure that no API routes are missing as a result of the reorganization
• Move the Swagger API docs page to /imswitch/api/docs
• Update URLs (e.g. for static assets and for API routes) in the Swagger API docs page
• Update URLs (e.g. for static assets and for API routes) in https://github.com/openUC2/ImSwitch-ReactAPP
• Ensure that the socket.io route is being used under /imswitch
• Ensure that any references to /libs/* (e.g. for imjoy) are updated appropriately
• Ensure that Jupyter notebook has a reasonable URL

This work is tracked on Notion at https://www.notion.so/Reverse-proxy-ImSwitch-on-port-80-via-our-ingress-proxy-28a4e612c78a80b6a78ce8938edaacd1?source=copy_link

[ethanjli]

Attached are the openapi.json contents (after human-readable reformatting) generated by FastAPI after the above commit (9d40424) from the current HEAD of the master branch (commit 856d94d):

• openapi 856d94d.json
• openapi 9d40424.json

From visually scrolling through every line of the diff between these two files, I have confirmed that no API routes have been lost with the above refactoring.

[ethanjli]

I haven't tested every single API endpoint, but I've tested things overall and they all seem to work. So I'm going to merge this PR now!