Add a "close" action to Drydock resources

Summary: This does nothing fancy, just closes the resource and releases/breaks leases. They'll get cleaned up in some to-be-written GC process.

Test Plan: Closed resources from web UI and CLI.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D3998