What has been done, 
- Add rich_text db evolution script. 
  - All old ReviewRequests, Reviews and Comments are marked as `rich_text = False`
  - All later ReviewRequests, Reviews and Comments will be marked as `rich_text = True`
- Enable markdown rendering with code highlight for all pre-text blocks
  - Use `marked` js lib render text
  - Use `google code prettify` for syntax highlighting
  - Rendered text are inserted along the page loading process using `document.write()`
- Use single template file to generate text blocks
- Always place text inside <pre> tag
- Adjust the css style for markdown text

Execute `./reviewboard/manage.py evolve --execute` on RB 1.7.6 database. 
- All reviews and comments are marked as `rich_text = False`

Then post a new review. 
- The new review is marked as `rich_text = True`
- The new review is rendered as Markdown with `.rich-text` class attribute
- The old reviews stays as they used to be