Refactor: Replace DOCUMENT_ constants with DTYPE_.

And not only final papers have documentType < 0.
Internal: Mark visibility, prefer dtype.
Rename optional document downloads.

Prefer "prefix-paper5-option.pdf" to "prefix-option5.pdf": better for
sorting. Also some refactoring.
Whitespace changes to schema.

Makes it closer to Code/backup.db --schema.
Use preg more wisely in unicodehelper.
Huh: \t is not \p{Zs}.
Default response word limit is 500.

800 is too much, says I after Eurosys.