Come up with better way of storing and generating control measurements

[hellais]

Currently control measurements are stored in a separate obs_web_ctrl table, however this presents challenges when needing to do cross table joins, which are not very efficient in clickhouse.

We might want to store them inside of the obs_web tables, but in doing so we should verify that it doesn't lead to any other unexpected consequences and will work well when implementing analysis.

Related to: #88