Added virtual RFID-Cards

[caco3]

Superseded by #319

This PR adds 5 virtual RFID-Cards.

It is based on the concept in https://forum.espuino.de/t/webradio-station-auf-eine-taste-legen/1343 but for now kept as simple as possible:

1. In settings.h assign any button action to one of the virtual RFID-Card cmds CMD_VIRTUAL_RFID_CARD_0{1..5}
2. In the WebUI, use the corresponding virtual RFIDs 90000000000{1..5} as the RFID number.

If the PR shall be accepted, I will also add some documentation to the WebUI.

Instead of using the numbers, the WebUI also could use a letter based RFID, eg. VIRTUAL_RFID_0{1..5}, how ever this would need extension of the parser in the WebUI and probably also in the firmware.

[caco3]

This is a mock-up of the UI:

@tueddy @biologist79 Please let me know if you would accept it and then I will implement it properly in the UI