[ticket/12352] Revert auth provider to db if default does not exist and check legacy hashes

This change will revert to the standard db auth provider if the one set in the config does not exist. Additionally, the db auth provider will now also check password hashes that are not in the format of the old phpBB3 and phpBB2 hashes if user_pass_convert is set.
The passwords manager will check hashes that do not fit our standard format using only the legacy drivers. Right now these are salted md5, phpass, and the newly added sha1_smf driver.
I'll also add WBB, MyBB, and vB3 legacy drivers if you guys are ok with the general idea behind this PR.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12352