[Security] Add challenge-response auth with scrypt verifiers and stop storing plaintext passwords

Challenge-response authentication: the client derives a scrypt verifier
(RFC 7914, EVP_PBE_scrypt, N=32768, r=8, p=1) and authenticates with
HMAC-SHA256(key, nonce), so neither the password nor its hash is
transmitted. The stored format becomes
"$scrypt$<n>$<r>$<p>$<salt>$<verifier>" and Response_PasswordSalt
now carries the cost parameters. Strict servers only accept scrypt
verifiers; legacy accounts are migrated after a successful login.

Fix #344 for challenge-response servers: a saved profile stores the
derived verifier under the password key instead of the plaintext password.
The connect dialog loads it without revealing it, autoconnect passes it
through, the change-password dialog no longer prefills the old password
field with it, and the client only persists the verifier when
"Save password" is checked.

Took 3 minutes

Took 1 minute

Took 10 seconds

Took 7 minutes

servatrice/migrations/servatrice_0036_to_0037.sql

@@ -0,0 +1,8 @@
+-- Servatrice db migration from version 36 to version 37
+
+-- The column must hold "$scrypt$<n>$<r>$<p>$<salt>$<verifier>" (up to ~255 chars) and arbitrary
+-- legacy base64 hashes, so it grows beyond the old 120-char size. varchar(255) is used as the
+-- column type is promotion-safe and avoids the row-format change ALGORITHM=INSTANT cannot do.
+ALTER TABLE `cockatrice_users` MODIFY `password_sha512` varchar(255) NOT NULL;
+
+UPDATE cockatrice_schema_version SET version=37 WHERE version=36;