Countdown: add a capability to decide who can Create

Summary:
This change adds the countdown.create capability to
allow to limit countdown creation.

This change comes from the Wikimedia Phabricator and
was originally proposed by v.bozzolan and implemented
by the kind user 20after4.

https://phabricator.wikimedia.org/T258599

Closes T15208
Ref T15081

Test Plan:

• go to the page /applications/view/PhabricatorCountdownApplication/
• play a bit with the "Can Create Countdowns" option
• note that it is respected

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Tags: #countdown, #policy_archived

Maniphest Tasks: T15208, T15081

Differential Revision: https://we.phorge.it/D25101