Add basic incident sort functionality

[noah-schechter]

Current
We offer three sort options in search: 1) recently modified, 2) alphabetical incident description, and 3) recently created.

Proposed
Users would also like to be able to sort by incident date.

Implementation notes
There are two design decisions here:

1. First, how to name these options concisely? I decided to use "Incident Date (Most Recent)" and "Incident Date (Least Recent)". There may be a better name for each option.
2. Second, how to order incidents without a date? I've gone ahead with Ecto's default sorting for null values, but there may be an option that is preferable here.

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