Comments on annotations

[josiahseaman]

Clicking on an existing annotation lets people add a comment. This is stored as its own annotation that mirrors the same start and stop. You may want some special cases here for condensing multiple comments or sending less data over the internet connection for "subordinate" annotations. The first phase can simply be to duplicate the start/stop/strand properties. These should definitely be stamped with a username if it isn't already.

Added to #172