Refactor some Herald code

Summary:
I was reading herald code for a task and realized that the method was
really long. So I refactor it to shorter methods.

Test Plan:
was still able to create a differential rule and commit rule; and
verified that dry-run still worked.

Reviewers: epriestley, tuomaspelkonen

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1077