Convert storage for Herald repetition policy to "text32"

Summary:
Depends on D18926. Ref T6203. Ref T13048. Herald rule repetition policies are stored as integers but treated as strings in most contexts.

After D18926, the integer stuff is almost totally hidden inside HeraldRule and getting rid of it completely isn't too tricky.

Do so now.

Test Plan:

• Created "only the first time" and "every time" rules. Did a SELECT on their rows in the database.
• Ran migrations, got a clean bill of health from storage adjust.
• Did another SELECT on the rows, saw a faithful conversion to strings "every" and "first".
• Edited and reviewed rules, swapping them between "every" and "first".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13048, T6203

Differential Revision: https://secure.phabricator.com/D18927