Issue due date changes without user input

Issue due date changes on its own, without any request from any user.

Example issue: https://gitlab.com/gitlab-com/marketing/field-marketing/-/issues/3014

Causes

see #329614 (comment 567374273).

Fixes

• Fix 1. Use defaultDate prop of GlDatePicker !60917 (merged)
• Fix 2. Disable autocomplete and render GlDatePicker after due date data fetching is done: !60973 (merged)

The above MRs stop (fix) due date changing without user input.

As a side effect of the above fixes, there is a minor UX glitch that should be fixed upstream in gitlab-ui.

• Fix 3. Use defaultDate as the initial date gitlab-ui!2192 (merged)

Follow ups

After "Fix 3" is merged, we should update feature specs.