More general API for assigning treatments to different subsets

[mizzao]

Treatments should be assignable to

• batches
• experiment instances
• user sessions ("assignments")

Moreover, treatment parameters should merge across these. A user that is in an instance within a batch should receive the combined parameters as a reactive variable on the client side. (The API for that could be fixed up as well.)

Original ideas:

• https://trello.com/c/OWt1a9VX/12-for-the-experiment-designer-how-to-write-the-meteor-app-to-show-different-interfaces-for-different-treatments
• https://trello.com/c/SLethuQL/57-rethink-how-treatments-are-merged-between-batches-instances-and-assignments

[mizzao]

User level-treatments implemented in 12d9390.