Add built-in Octoprint support (v2)

[ayufan]

This is based on #343. It removes a lot of limitations of #344.

This adds a built-in support for Octoprint / Klipper+Moonraker setups allowing to directly
upload and start print. It stores data in browser and sends requests directly to the install.

Life is great compared to previous take #344:

• It uses a POST form, so it does not require CORS
• It does not show a great info once uploaded, as it shows raw JSON from the other service directly in browser
• However, it works OOB without significant extra configuration

Test it here:

• https://ayufan-research.github.io/teachingtechYT.github.io/calibration.html

[ayufan]

(I noticed some problems with this. Will update comment).

It appears that:

• it works always fine when octoprint/moonraker is on https://
• when using http:// it works fine on Safari/Firefox which shows dialog box about unsecure connection
• when using http://it does not send a proper file on Chrome which shows a full page prompt that you need to Send anyway

[mikekasprzak]

Nice! I was just coming to see if anyone had added something like this (though I was checking for raw GCODE over serial).