espalarm Code

Make function public for reuse.
Change feedback to the app to include armed status.

Include support to save FCM device keys on LittleFS storage, and have the phones register
themselves remotely via a secure JSON query.

Remove hardcoded cert dead code

Send FCM message to a channel.
Add a bit of doc.

Remove whatsapp some more

Replace whatsapp by firebase cloud messaging.
Conditional compilation to make for a non-OLED communication device.
Replace dependency on one root certificate by cert bundles.

build: small cleanups
JsonServer : handle an uncaught error, removes a compiler warning.
WebServer: simple list of sessions added, see previous change.

Simplistic version of code that'll cope with more than one SSL connection.
See Readme.md and https://github.com/espressif/esp-idf/issues/8288 , esp-idf-v4.4 plus
a fix are needed

Remove dead code related to the pre 4.4 security code

API change to support esp-idf-v4.4-rc1.
Document that esp-idf-v4.4-rc1 now supports the additional functionality we require.