Share schedule back feature

Resolves #284

added functionality to the UI for sharing back schedules to friends that the user has schedules of

[x] Extract the reference passed to the input div of the modal and store in the component which will trigger the modal instead. Whenever the user clicks share back, we will pass in the email of the friend that they're trying to share their schedule with to the modal using the ref that we extracted. This is a suggestion, feel free to use other solutions which fulfill this goal. When the invitation modal opens, it will have the email of the friend already input in the box and they will just have to press send.
[x] Create the UI elements as displayed in the figma prototype. However instead of only having one of the share back options at a time as shown in the pictures above, have both of them displayed at all times for now.

Open compare schedules with friends not shared with

gt-scheduler / website