24 simplify sql schema

Today we are using a ManyToMany field for timelines in a sketch. This is unnecessary complexity and makes migrations hard. Let's change this to be a normal foreign key relationships on the SketchTimeline model instead and add the timelines attribute as a property on the Sketch model.

NOTE: This is one of several CLs for a overall refactor/clean up of the code base. This one is just dealing with the schema migration stuff, not any overall clean up. This will be done in another CL. See the following gist for a rough roadmap:
https://gist.github.com/berggren/3888c6ebb128f6adc63d

Reviewer: @joachimmetz