Lock relay status on boot

[mcspr]

resolves #1697

• relay_t::lock flag to control which target status we want. after relay changes to that status, we can no longer change it
• boot mode setting to lock relay in either OFF or ON state

cc @goatchurchprime @xoseperez
this might interfere with planned motor changes from #1510 ?

[mcspr]

cc @lancelon
this will lock the relay instead of disabling the button. meaning, you can still receive button events if you want that
edit: PR adds the underlying api. meaning, we can also add something else (MQTT, HTTP API calls) to set lock. it is not only for boot modes

and plugging #1680 again, where you can modify hardware preset and just remove button gpio in config. also, see #1505 (instead of #1322 proposal).

[xoseperez]

I would add some visual notification to the web UI when the switch linked to the button is locked. Like disabling the toggle in the UI...

[mcspr]

"input[disabled]" does just that.
However, if lock becomes togglable, message should go with relayStatus instead. Maybe even nested within new relayState: {status: [...], lock:[...]}, since processData can encounter lock before any UI is initialized (can't depend on proper key ordering)