migrate.py should fix known errors in the classic mail parsing

The migrate.py script should fix various known errors in the classic email parsing algorithm.
For example:

- remove line-wraps and other optional white space from header fields, especially Message-Id and References
- fix up mbox date values that don't agree with the epoch

Start by reading migrate.py and tracing how it handles the classic email parsing algorithm. Reproduce the listed cases for folded header whitespace, especially Message-Id and References, and mbox dates that disagree with the epoch; done means those known errors are corrected during migration.

Written by the indexing model from the issue text.