Adding Enum to ProposalComment

[kracekumar]

In Junction, there are three type of comments, comments by all users (public), private comments (only visible to reviewer and proposer), notes (reviewers internal discussions). These are managed by two flags, https://github.com/pythonindia/junction/blob/master/junction/proposals/models.py#L153.

It is source of confusion to look up multiple flags. The better solution is to have enum aka PositiveIntegerField and query accordingly.

Having said this, this feature requires careful handling of migration.