Open and import password-protected PDFs

Password-protected PDF attachments now show a clear "password-protected" notice instead of locking the page up — but there's still no way to get at the contents.

Add a way to unlock these files: prompt for the password when previewing or importing, then store an unlocked copy so the document behaves like any other — preview, search, redact and export.

Raised off the back of the locked-out display bug report.