ui: add workflow sharing modal

[DaanRosendal]

To do and discuss

• [x] Message sending still needs to be implemented.
  • This has been discussed and will involve notifications in the platform and emails
• [ ] Save sharing messages to the database or send and forget?
  • [ ] If we decide to send and forget, the database model can be updated to remove the message.
• [x] Should we let users specify a time that the sharing expires, or is a day enough?
  • Day is enough

The wireframe depicted in the corresponding issue has been mimicked as closely as possible:

Closes #368