Add support for creating new discussion

[David Jones]

Currently Canvas Discussion style only works to connect to an existing discussion.

Add support for it to create a new discussion

• Figure out how to handle style naming
  • add a Canvas New Discussion
  • add a Canvas Existing Discussion style (matching current Page behaviour)
  • Have word2canvas check if a discussion is new or existing
• Have the module check indicate difference between new and existing discussion
• Handle creating the difference