Add setup check for Cloudflare proxy usage detection

Benefits:

• Inform admins that they may have upload problems (not working and/or poorer performance) due to chunk sizing and timeouts
• Inform admins that they may experience problems that will not be supported (if RocketLoaderr is enabled) such as login issues and unexplained application problems

Proposed checks:

• INFO when detected[^1].
  • transaction size limits (i.e. upload chunk size limited to 100MB)
  • transaction time limits (impacts chunk assembly)
  • always remember to pause/disable when troubleshooting front-end issues
• ERROR when Rocket Loader is detected[^2]
  • must be disabled with Nextcloud
  • breaks things (logins, calendar, etc)

[^1]: We can detect the usage of Cloudflare [reverse] proxy mode by the presence of the cf-ray HTTP header
[^2]: When CF is detected to be in-use, we can check for the presence of rocket-loader.min.js added /login via <script> and/or check for ajax.cloudflare.com being added (by CF) to Content-Security-Policy

Start by tracing the setup-check flow and the /login response, using the cf-ray header, rocket-loader.min.js, and Cloudflare additions to Content-Security-Policy as the detection points. Done means administrators receive an INFO warning for detected Cloudflare proxy limits and an ERROR when Rocket Loader is detected.