The 'prepopulated-autocomplete-mention' plugin takes care of annotating @mentions
with immutable entities and styling them a bit when the editor loads, in order to
decorate mentions that could be present in the prepopulated text and to make them
behave exactly like mention entities that are generated by draft-js-mention-plugin.

Mentions written by users after the editor loads are handled by draft-js-mention-plugin.

Two strategies coexist: prepopulatedMentionStrategy is used to locate mentions to be
annotated with entities in the text, and prepopulatedMentionEntityStrategy is used
to locate mention entities in order to style them.
