This change adds a new RBTools command for creating and editing reviews.

This uses the new MultiCommand subclass in order to provide a variety

of functionality within a single command:

rbt review edit will create or edit a draft review.
rbt review discard will discard a draft review.
rbt review publish will publish a draft review.
rbt review add-diff-comment will add a new diff comment.
rbt review add-file-attachment-comment will add a new file

  attachment comment.
rbt review add-general-comment will add a new general comment.

Based on work by Anahita Mohapatra at /r/11502/.

Ran through manual test cases for each of the review subcommands:

- Creating a new review with various content.

- Editing an existing draft review.

- Creating general comments (both markdown and plain)

- Creating file attachment comments (both markdown and plain)

- Creating diff comments (markdown and plain, single- and multi-line).

- Discarding a draft review.

- Publishing a draft review.

- Tested validation and error handling (file attachment IDs, diff

  filenames, diff revisions)