This replaces 'plugins/omemo' with 'plugins/omemo-views' and
'headless/plugins/omemo'. Most of the original code is still in
the former, but the contents of 'util' has been moved to the
headless version. Still pending: moving 'index.js' and all the
other files in the plugin!

The 'omemo-views' plugin is still there, but most of the logic has
now been moved to the new headless 'omemo' plugin, with only the
UI part remaining in 'omemo-views'.

Turns out there was no headless component to these functions, so
nothing to be moved!