Fetch cycle analytics stage and median data from dedicated endpoints

⚠️ Requires:

Fetch groups stages and events: !18514 (merged)
BE: !18991 (merged)

For #13076 (closed) are moving to new /-/analytics endpoints for fetching the required cycle analytics data, currently median values for each stage are returned in the initial cycle analytics data request, but will eventually move to individual requests - GET */cycle_analytics/median/:stage_id

Once the stages have loaded (REQUEST_GROUP_STAGES_AND_EVENTS) we should request the median for each stage
Add mutations to update the stages with their median value
Update endpoint for fetching the events for the currently selected stage

Edited Oct 29, 2019 by Ezekiel Kigbo