This patch adds the finishing touches to support for general comments

-- that is, comments unattached from a file attachment, diff, etc. --

on reviews. 
A button has been added to the review dialog to add a general comment.

These comments can be deleted with the delete comment icon that is

present with all other comments. JavaScript unit tests have been added

to test the deletion functionality.
A button has been added to the draft review banner to add a general

comment. This comment can only be deleted from within the review

dialog.
The include_text_types fields have been removed from the

BaseCommentResource because it requires special parsing. They were

added, but their type was incorrect. The field expects a

comma-separated list of the text types, not a single text type. This

broke the ability to comment has so has been reverted. A unit test has

been added to ensure this behaviour does not regress.
Calls to _no_access_error in the ReviewGeneralCommentResource and

ReviewReplyGeneralCommentResource have been corrected to

get_no_access_error, as the former method does not exist.

Ran unit tests.
Ran JS tests.
Added comments from both the banner and the dialog.
Deleted comments from the dialog.