Jebeck/move edited message

Tideline changes necessary to move a message whose timestamp has been edited.

Needs to be merged in concert with [blip PR #101](https://github.com/tidepool-org/blip/pull/101).

@ianjorgensen might be nice if you had a look. The refactoring in `message.js` is something I'd like to keep doing everywhere (i.e., datatype.xPosition, etc. functions).