Add Commenting

[zippy]

When in the view mode of a document users should be able to select text in a section and create a comment about it.

• comments are implement creating a document of type _comment following some conventions: the comment's text is stored in a section named "comment" and (if any) a suggested change in a section named as "suggestion" with the suggested text, and the following in the meta to identify what is being commented on:

  document: EntryHash, // points to document being commented on
  section: EntryHash, // section being commented on
  start_index: u32, 
  end_index: u32,

• comments shouldn't show up as separate documents in the alignment box, but rather on the right margin of the document as is typical in gdocs etc.
• a reply to a comment is just a comment that has a document hash that references another comment. Note that a reply might comment on either the comment or the suggestion.
• an editor should be able to mark a comment as addressed.