On-call schedules frontend cleanup

• Use apollo cache to update the page on schedule. add/edit/delete same with rotations - create/delete sometimes works as expected - maybe something is wrong with cache policy => !52773 (merged)
• Restore empty grid view - currently, we have only a header when there is no rotations, but at some point, there was an empty grid => !52917 (diffs)
• Generate color for each participant of the rotation (currently hardcoded to some shade of blue) => !52839 (merged)
• Decide with UX how to adjust tiny shifts on large timeframes - as they are really tiny - maybe not to show name/avatar at all, as well as tooltips
• Also, I think there is an issue with positioning shifts on WEEK's timeframe - it should be easier to see when each shift has a dedicated color => !53524 (closed)
• Swap preset type dropdown to button group => !52773 (merged)