Web-UI: Replace the template processor (#253)

* remove template-processor

* fix showing values (slider, maxLength)

* separate JSON settings into sections (neopixel, battery)
Make "Reset" values to factory settings working

* bugfix saving inactivityTime & always loading defaults

* fix compiling with both BATTERY_MEASURE_ENABLE & HALLEFFECT_SENSOR_ENABLE enabled

* set to LOGLEVEL_ERROR if saving settings failed

* Enabled property not needed: if feature is disabled do not provide the JSON object

* rename JSON variables for better readability

* wifi global settings

* changes suggested by @SZenglein review:
- refactor server-side hostname validation to Wlan.cpp
- rename JSON "neopixel" -> "led"
- rename getSettings() -> requestSettings()
- bugfix MQTT max-length
- bugfix save "scan_wifi_on_start"
- sync with latest DEV

* load SSIDs via websocket, avoid two extra HTTP requests