Add security review processors

Related to #5 (closed), this is a partion and follow-up MR.

Depends on

Add security reviewer role

Changes

Add three processor:

1. SecurityReviewReady: A processor while already for security review, will remind security reviewer and add ~security-review::pending.
2. SecurityReviewApprove: Resolve security discussion thread and add ~security-review::approved
3. SecurityReviewDisapprove: Unresolve security discussion thread and add ~security-review::pending