Generic RDFa support

The editor should make it easy to add rdfa annotations, both before and after text input.

There is a plugin that is no longer used that sort of offers this: https://github.com/lblod/ember-rdfa-editor-generic-model-plugin/blob/master/README.md

This plugin was seen as too complex for most users (i.e. a power user tool). We should aim for a new plugin that provides similar functionality but makes this more user friendly.

A user should be able to select a piece of content (plain text or multiple elements) and wrap that in an rdfa annotation.

Ideally the plugin is aware of any cardinality constraints so it can inform the user if required annotations are missing and guide them in adding those.