mail: allow PLAIN auth over non-tls connections (#3900)

This allows mattermost to use a non-tls connection with a SMTP server that
supports PLAIN auth (but not LOGIN). The go library explicitly allows PLAIN
auth over non-tls connections - https://golang.org/src/net/smtp/auth.go#L55

Fixes #2929

webapp/components/admin_console/connection_security_dropdown_setting.jsx

@@ -27,6 +27,20 @@ const CONNECTION_SECURITY_HELP_TEXT = (
                     />
                 </td>
             </tr>
+            <tr>
+                <td>
+                    <FormattedMessage
+                        id='admin.connectionSecurityPlain'
+                        defaultMessage='PLAIN'
+                    />
+                </td>
+                <td>
+                    <FormattedMessage
+                        id='admin.connectionSecurityPlainDescription'
+                        defaultMessage='Mattermost will connect and authenticate over an unsecure connection.'
+                    />
+                </td>
+            </tr>
             <tr>
                 <td>
                     <FormattedMessage
@@ -66,6 +80,7 @@ export default class ConnectionSecurityDropdownSetting extends React.Component {
                 id='connectionSecurity'
                 values={[
                     {value: '', text: Utils.localizeMessage('admin.connectionSecurityNone', 'None')},
+                    {value: 'PLAIN', text: Utils.localizeMessage('admin.connectionSecurityPlain')},
                     {value: 'TLS', text: Utils.localizeMessage('admin.connectionSecurityTls', 'TLS (Recommended)')},
                     {value: 'STARTTLS', text: Utils.localizeMessage('admin.connectionSecurityStart')}
                 ]}