get rid of LIGHT_CHANNELS, but keep channel setting for the my92xx
also clean up recent changes, channel command did not actually work

amend bdbd127bfb

Make sure 'custom provider' `lightAdd()` scheduled function locks further channel modifications,
and take care of light sanity checks before the light update is even scheduled.

Reworked rtcmem reader / writer to use default values, instead of accidentaly using
the unitialized mem of the stack. Also, reworked const names used by the lights module
to distinguish them from the build flags.

fixes #2419

Make sure transition is always pushed into the vector the same way

Only happens with ESPURNA_BUILD_SINGLE_SOURCE=1:

/home/runner/.platformio/packages/toolchain-xtensa/xtensa-lx106-elf/include/c++/4.8.2/bits/vector.tcc:405:31: error: __c causes a section type conflict with __c
    _M_check_len(size_type(1), __EXCSTR("vector::_M_emplace_back_aux"));
                               ^
/home/runner/.platformio/packages/toolchain-xtensa/xtensa-lx106-elf/include/c++/4.8.2/bits/vector.tcc:71:23: note: '__c' was declared here
  __throw_length_error(__EXCSTR(__N("vector::reserve")));

- make sure we have custom provider before calling the update,
replace old check in the update function
- fix race condition in the provider function, causing complete lock-up of
lights controls (triggered by sending light updates very fast. for example,
using arrow keys with the webui sliders; ref. #2424)
- rework state change functions to work using loop() func instead of a
pair of scheduled functions. we should not go out-of-sync now, and it
also avoids forcibly creating std::func object each invocation
- rework other providers to be more in line with custom callbacks -
introduce state, value and update calls as a common way to handle lights
- correctly handle state change when turning OFF and ON
- update tuya to store the last state, never send channel values when OFF (ref. #2424)

also, clean-up PGM_P usage and just use `const char*` instead

- fix argc checks in state
- use ctx as output for commands
- show state when calling without args

inline-source uses 'image/ico' otherwise
supposed to be fixed upstream via the usage of type="image/x-icon" as
link attribute, but there's no npmjs release yet

ref. #2423

ref. #2423

amend c100696f7b

- retain setting
- generate topic and message in the discovery object instead of
creating them all at once
- re-implement relays, lights and sensors discovery
- rework name normalization, use relative magnitude indexes
- replace original lights code with schema: json, subscribe to a special
topic to handle lights instead of using the common API
(still wip - colors, hs, temperature settings)

resolve #630
resolve #1564
resolve #2403

- simply use a basic {String, String} struct
- fix MQTT_TOPIC_TIME vs. MQTT_TOPIC_DATETIME
- get rid of unused chaining and indexes

- shrink utils source file, move heartbeat and boot management into system
- improvise with 'heartbeat' functionality. include scheduler implementation that will
manage the per-module heartbeat callbacks with individual 'mode' and
'interval' settings. current ones are mqtt (including relays, lights, thermostat), debug and
influxdb. preserve heartbeat NONE, ONCE and REPEAT, REPEAT_STATUS is effectively a hbReport & status bit.
- mqtt heartbeat is managed through mqttHeartbeat() callbacks
- tweak mqtt callbacks to use lists instead of the vector, slighly reducing the size of the .bin
- update WebUI, include report setting and update hbMode values
- make sure general.h settings include new heartbeat,
move constant definitions outside of the header
- correctly include dependencies through the .cpp, avoid leaking
internal details.
- as a side-effect, base headers are no longer included recursively

- for some reason old key was default-constructible
- fix magnitude key generation

- specialize ntp functions to split time_t into 32bit ints when time_t is larger than rpn_int
- rework 'tick' variable into an operator that can be triggered once
- make sure operators expect new time_t split via argc of 2

- make sure server is never empty
- disable dhcp option when server *is* empty
- update configure() to detect both tz and server changes