R1:8ddf883d2ee1

Cut Herald rules off at 1GB of diff text

Summary:
Ref T4276. When a change is larger than 2GB, PHP can not read the entire change into a string, so Herald can not process it.

Additionally, we already have a time limit for practical reasons, but it's huge (probably incorrectly). To deal with these things:

- Add an optional byte limit to `diffusion.rawdiffquery`.
- Make the query with a 1GB limit.
- Reduce the diff timeout from 15 hours to 15 minutes.
- Add a "Changeset is enormous" field. This…