feat: simplify priority handling

[creightonfrance]

• [Feature Request] Rework priority handling #1128

Changes

• Sidebar will now present the priority value as a dropdown (High, Medium, Low)
• Priority changing from task list item goes through the following states: 0 => 1 => 5 => 9 => 0

TODO

• Unit test (holding off, anticipating change requests and file conflicts with feat: allow editing completion date #2686)

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