Extension modal refactor

[ublefo]

List of changes

• Add input validation and clearly communicate the length requirement for the reason field
• Replace the length slider and switch to a date picker to improve UX
• Add red asterisks to upload-submission-modal as a quick fix to indicate mandatory fields

Notes

Date picker range logic:

• Start date is tomorrow of target date if not overdue, or tomorrow of current day if overdue.
• End date is always the due date (due_date on the backend, the hard deadline).
• The component calculates the number of weeks and rounds it up before sending the request to maintain compatibility with the current backend logic. The extension granularity refactor (weeks -> days) will be rebased on top of this.

Screenshots

Reason not provided:

Reason not long enough:

The current target date (displayed as due date on the frontend) is 14 May, available range starts at 15 May:

The due date (hard deadline) is 18 May, so the extension can only be selected up to that day:

Overdue task, target date is 2 Apr, starting date in the date picker is the next day after today (1 May), since it doesn't make sense to request extensions to a past date:

[jakerenzella]

Will do a test when it's all merged with the day changes