srml/authority-discovery: Abstract session key type (#3698)

* srml/authority-discovery: Abstract session key type

Previously `srml/authority-discovery` dependet on the `srml/im-online`
session key type directly. With this patch `srml/authority-discovery` is
generic over the session key type it is going to use, as long as it
implements the RuntimeAppPublic trait.

With this patch one can use the `srml/authority-discovery` module
without the `srml/im-online` module.

Next to the above, this patch configures `node/runtime` to use the babe
session keys for the authority discovery module.

* srml/authority-discovery: Fix line length

* srml/authority-discovery/Cargo: Move babe to dev-dependencies

* node/runtime: Bump implementation version

* srml/authority-discovery: Add doc comment for authority discovery Trait