Timeslots with no location cannot be edited

[jennifer-richards]

Describe the issue

In the workaround described in #4592, timeslots with no location are created. The timeslot editor assumes that all timeslots are connected to a room. Since this is apparently not enforced at the database/model level, there should be a method for finding/editing timeslots with no location.

Fixing #4592 will make this less urgent because timeslots all have locations in the intended workflow.

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