embargo: fix date update bug and add helptext to date field

[jennur]

Closes https://github.com/inveniosoftware/react-invenio-deposit/issues/561

• Adds the correct object-property reference so that the date is updated correctly in the access-message
• Added help-text to the date field to improve ux. (The placeholder disappears when you start typing.)

Add PR to apply some spacing inconsistency fixes: https://github.com/inveniosoftware/invenio-app-rdm/pull/1840

Screenshots