R1:7ed35123a347

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…