Allow setting/disabling Emergency Stop during setup/via Serial commands

[nullstalgia]

Allows enabling/disabling EStop without needing to change the compile-time constant.

Default pin is still obtained via OPENSHOCK_ESTOP_PIN

Using a pin of 0 (GPIO_INVALID in Common.h) in either the Captive Portal or via Serial will disable the EStop manager.

Remaining tasks:

• [ ] Captive Portal formatting seems to be affected, need to fix.
• [ ] Need to further test updating to this from old firmwares (so far, works correctly)