Expose initially set values of Maniphest custom date field and point values when removing them

Summary:
After removing or resetting a custom date field value or points value initially set at task creation there is no way to find and restore the previous value.
As Phorge adds a transaction entry anyway, include the previous value in the transaction title.
This makes it easier to revert an accidental removal of such data and is also more aligned with the behavior of other custom field types.

Closes T15846

Test Plan:

• Define a custom date field in Maniphest via http://phorge.localhost/config/edit/maniphest.custom-field-definitions/ and also enable http://phorge.localhost/config/edit/maniphest.points/
• Create a task with those fields set to non-custom values via a create task form which allows editing/changing these values
• Edit the task and remove/reset the values of these custom fields
• Look at the task and its transaction entries

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15846

Differential Revision: https://we.phorge.it/D25678