Fix support for upgrading encrypted wikis

Importing an encrypted wiki ordinarily doesn’t place the password in
the password vault on the basis that one ought to be able to import
from a file without automatically inheriting its password.

Now there’s a configuration option that can be used by the upgrade
plugin to cause the password vault to be updated with any password
entered by the user. The end result is that the user only needs to
enter their password once.
Docs update
Styling for upgrade wizard
Fix typo