Markdown and Linkify should not add nofollow to relative links and trusted domains

[jace]

rel="nofollow" is currently added to all links by markdown and sanitize_html. It should not be added to:

1. Relative links
2. Trusted domains as specified in the app configuration, which requires both functions to consult current_app or to receive the domains as a parameter. If the latter, MarkdownColumn must pass this in.