remove SCRAM-SHA-256 from advertised SASL mechanisms

Advertising SCRAM-SHA-256 breaks irccloud, which doesn't fall back to PLAIN
if it sees SCRAM advertised but SCRAM then fails (as is the case for any
account password hashed on Ergo 2.7 or lower).

Leave a config option for irctest to enable it in the controller.

irc/config.go

@@ -303,6 +303,7 @@ func (t *ThrottleConfig) UnmarshalYAML(unmarshal func(interface{}) error) (err e
 type AccountConfig struct {
 	Registration          AccountRegistrationConfig
 	AuthenticationEnabled bool `yaml:"authentication-enabled"`
+	AdvertiseSCRAM        bool `yaml:"advertise-scram"` // undocumented, see #1782
 	RequireSasl           struct {
 		Enabled      bool
 		Exempted     []string
@@ -1379,7 +1380,12 @@ func LoadConfig(filename string) (config *Config, err error) {
 		config.Accounts.VHosts.validRegexp = defaultValidVhostRegex
 	}
 
-	config.Server.capValues[caps.SASL] = "PLAIN,EXTERNAL,SCRAM-SHA-256"
+	saslCapValue := "PLAIN,EXTERNAL,SCRAM-SHA-256"
+	// TODO(#1782) clean this up:
+	if !config.Accounts.AdvertiseSCRAM {
+		saslCapValue = "PLAIN,EXTERNAL"
+	}
+	config.Server.capValues[caps.SASL] = saslCapValue
 	if !config.Accounts.AuthenticationEnabled {
 		config.Server.supportedCaps.Disable(caps.SASL)
 	}