Allow us to record conversion events for experiment participants

Adding on to the basic concept outlined in #255615 (closed) of knowing if a user has performed the desired conversion action for an experiment or not, we would also like to know when the user performed that action (especially as it relates to when they first joined the experiment).

To keep track of this historical data, we will add a converted_at timestamp column (e.g. 2020-09-25 17:11:42 +0000) to the experiment_users database table. Before the conversion event the value would simply be NULL (and the corresponding value in the converted boolean column would be false). After they perform the conversion event, the value of the converted_at column will be stamped with the time of the conversion event (and the corresponding value in the converted boolean column would get updated to true). Then we can do comparisons with the already-present created_at timestamp column in the experiment_users table to see how long it took from when the user initially joined the experiment to when they performed the desired conversion event.