Allow "!accept" to be enabled through configuration

Summary: For reasons explained in the config I've omitted this from the default
action set, but it's trivial to support it. See D916.

Test Plan: Commented on a revision, was informed I could "!accept" in the email.
Used "!accept" to accept the revision.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 928