a11y: Add aria-label to calendar date picker button
Summary:
Form elements should have associated labels per https://www.w3.org/WAI/tutorials/forms/labels/
Ref T16072
Test Plan:
Have a custom field of `"type": "date"` defined in `/config/edit/maniphest.custom-field-definitions/`.
Go to edit a task (`/maniphest/task/edit/1/`) whose form exposes that custom field.
Inspect the `<a>` field parameters with `class="calendar-button"` and see an additional `aria-label="Calendar date picker"`.
Optionally, run an Accessibility check in Chromium Lighthouse.
Reviewers: O1 Blessed Committers, chris
Reviewed By: O1 Blessed Committers, chris
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16072
Differential Revision: https://we.phorge.it/D26046