Fixes T10330.

• Anywhere we support "matches regexp", also allow "does not match regexp". Although you can sometimes write a clever negative regexp, these rules are better expressed with "does not match <simple regexp>" anyway, and sometimes no regexp will work.
• Always allow "does not contain" when we support "contains".
• Fix some JS issues with certain rules affecting custom fields.

• Wrote an "Affected files do not match regexp" rule that required every diff to touch "MANUALCHANGELOG.md".
• Tried to diff without the file; rejected.
• Tried to diff with the file; accepted.
• Wrote a bunch of "contains" and "does not contain" rules against text fields and custom fields, then edited tasks to trigger/observe them.
• Swapped the editor into custom text, user, remarkup, etc fields, no more JS errors.