Reimplement previously removed "standard sensors"

[mairas]

There are ~four~ five system information sensors, previously called standard sensors, that were enabled by default and could be triggered on or off using SensESPAppBuilder:

• SystemHz
• FreeMem
• Uptime
• IPAddrDev
• WifiSignal

SensESPAppBuilder needs to have convenience methods to enable these:

• enable_system_hz_sensor()
• enable_free_mem_sensor()
• enable_uptime_sensor()
• enable_ip_address_sensor()
• enable_wifi_signal_sensor()

And additionally, one convenience method to enable all of these. My proposal for the name:

• enable_system_info_sensors()

[ba58smith]

I like system_info_sensors.

[JohnySeven]

Like it! Library user have it all under control!

[mairas]

Fixed in #472.