mooclet frontend: reward metric section in stepper

[danoswaltCL]

A mooclet (adaptive) experiment will always need to have a reward metric. For MVP we will always be creating this metric for the user, and attaching a query for success percentage automatically. See #1934 and #2191 for how the backend will wire this up.

The convention for this metric key will be the [UPPERCASE name of the experiment] + _REWARD. A mooclet experiment will be guaranteed to be uniquely named (#2199), so this will suffice as a unique key.

Requirements:

• For mooclet experiments only, a read-only "Reward Metric" section will appear below the usual Metrics in the metric tab.

• The reward metric key will be sent on the experiment as rewardMetricKey

• The link is TBD, we will need a story to create documentation link in gitbook.

[kawcl]

Updated requirements to include key on experiment dto.