Change data model to store measurements by themselves

[angles-n-daemons]

Treatments are manually assigned measurements right now when measurements are given. This is done via a hacky most recent lookup mechanism and takes up space because each measurement needs to be duplicated for each active experiment treatment.

I have since realized I can do reporting on measurements if I filter them by the dates of the experiments and join them onto the treatments user ids to associate a variant. This is a more scalable and reasonable solution in my mind.

Metric table should now have an index on experiment & date.