Mi32 legacy: refactoring, bugfixes, generic device scanning

[Staars]

Description:

• remove some old unused code fragments
• minor optimisations reducing flash space usage and avoiding unnecessary copy operation
• limit sensor number to 32 for current web UI code
• show clock in the web UI (as a friendly reminder to check, if the timezone of the ESP is correctly set)
• dynamically update number of observed devices in web UI
• show timestamp of last received packet in web UI (as date string) or in the JSON payload at teleperiod (as unix time).
• add mi32option5 0/1 to turn off/on observation of any BLE device with a public address, which will be reported with it's name from the BLE advertisement (if existent) or with the naming scheme BLE_x, where x is the internal sensor number. Will report BLE payload as hex string. Web UI will show graph over the last 24h for a rough overview of sightings.

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.3.0.0
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass