Add mechanism for moving state to 'verified' on merge.

[bblancha]

Certain workflows call for doing additional manual testing while a change is in PR, before that PR can be merged.
In these workflows, it is preferable to immediately move a jira to state "verified", once the PR is merged.

A user should be able to write a comment or add a label that causes octobot to verify tickets that would normally be moved to resolved state when the PR is merged.