Complete default editor toolbar: support comments

Context

In #637 we will introduce the ability to customise the tools available in the editor within Ketty. Before we do that, we need to complete the default toolbar.

Proposal

• Enable comments
• Display username with comment
• let user @mention another user who has access to the book
• Send email when user is mentioned in a comment

Design

Implementation (if applicable)

Alternative approaches (if applicable)