Prevent the creation of multiple schedules in the backend #1782

Describe the bug

While we don't have official support for multiple schedules (until #722 is done), we should prevent multiple schedules from being created. There are multiple places were we are assuming there's just one schedule so this might be the source of multiple hidden bugs (related PR #1777)

To Reproduce

It is possible to reproduce this by making direct API calls to the create schedule endpoint but also we should probably check the touchpoints where a schedule is created (FTUE) / updated (Availability settings).

Expected behavior

Only one schedule should exist for a user.

Actual behavior

There are several users that unknowingly have multiple schedules.