Mentions only initialise when the user focuses the field

All fields that support user mentions are called "mentionable" fields.

Mentionable fields are initialised at one of two times:

• When DomContentLoaded occurs, or
• When the field is focussed by the end-user

This means that some mentionable fields have a chance to not get initialised if:

• The field did not exist in the DOM at the time DomContentLoaded occurred, and
• If the field is programmatically focussed, as opposed to focussed by the end-user.

This happens to be exactly what the "add comment" field does.