TaskScheduler doesn't order tasks correctly

[Minusome]

Bug Summary:

TaskScheduler doesn't rearrange existing tasks when adding a new task with an earlier due date .

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Steps To Reproduce

1. Add a task A with a later due date (e.g. 3 hours in the future)
2. Add a task B with an earlier due date (e.g. 1 hour in the future)

Expected behaviour

task B should be ordered before task A.

Observed Behaviour

task B ordered after task A.

Screenshots

Environment

• Version: latest
• Operating System: macOS
• Browser (if any): Chrome

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[Minusome]

Note that due dates are not yet persisted on the server.
Blocked by #273 which will probably add it.