Pre-enable OMEMO plugin

[stokito]

The OMEMO is built-in feature for most XMPP clients to simplify its usage.
The Psi has the OMEMO as a separate plugin that needs to be enabled.
It would be great to make the OMEMO plugin pre-enabled or even made as a part of core.

[Neustradamus]

To follow this important ticket.

It will be nice to have a perfect OMEMO: "urn:xmpp:omemo:2"...:

• https://xmpp.org/extensions/xep-0384.html

There are a lot of other OMEMO tickets too...