Fix editing a Calendar import ICS URI

Summary:
Before this change it was only possible to create a Calendar import ICS URI.
After this change it's possible to also edit already-existing elements.

This change fixes this specific exception when visiting similar pages:

/calendar/import/edit/5/

Argument 2 passed to PhabricatorCalendarImport::initializeNewCalendarImport()
must be an instance of PhabricatorCalendarImportEngine, null given, called in
phorge/src/applications/calendar/editor/PhabricatorCalendarImportEditEngine.php
on line 45

Before this change it was only theorically possible to edit the name, policies, etc.
but not the URI.

This change also introduces the ability to edit the specific ICS URI,
in order to change legitimate parts of the URI, like the secret token.

Closes T15137

Test Plan:
I tested in my own installation with success lints.

To test, visit the Calendar, create an import ICS URI, and then try to edit it again.
It will work only after this change.

I was not able to conclude the "arc diff" since it tries to connect
to an unexisting database owned by root.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, Cigaryno, speck, tobiaswiese, Matthew

Maniphest Tasks: T15137

Differential Revision: https://we.phorge.it/D25061