[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/servatrice.sql

@@ -20,7 +20,7 @@ CREATE TABLE IF NOT EXISTS `cockatrice_schema_version` (
   PRIMARY KEY  (`version`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 DEFAULT COLLATE utf8mb4_unicode_ci;
 
-INSERT INTO cockatrice_schema_version VALUES(36);
+INSERT INTO cockatrice_schema_version VALUES(37);
 
 -- users and user data tables
 CREATE TABLE IF NOT EXISTS `cockatrice_users` (
@@ -28,7 +28,7 @@ CREATE TABLE IF NOT EXISTS `cockatrice_users` (
   `admin` tinyint(1) NOT NULL,
   `name` varchar(35) NOT NULL,
   `realname` varchar(255) NOT NULL,
-  `password_sha512` char(120) NOT NULL,
+  `password_sha512` varchar(255) NOT NULL,
   `email` varchar(255) NOT NULL,
   `country` char(2) NOT NULL,
   `avatar_bmp` mediumblob NOT NULL,