KEYCLOAK-4383: Dbl-click login button leads to "already logged in" page (#4614)

themes/src/main/resources/theme/base/login/login.ftl

@@ -6,7 +6,7 @@
         ${msg("loginTitleHtml",(realm.displayNameHtml!''))?no_esc}
     <#elseif section = "form">
         <#if realm.password>
-            <form id="kc-form-login" class="${properties.kcFormClass!}" action="${url.loginAction}" method="post">
+            <form id="kc-form-login" class="${properties.kcFormClass!}" onsubmit="login.disabled = true; return true;" action="${url.loginAction}" method="post">
                 <div class="${properties.kcFormGroupClass!}">
                     <div class="${properties.kcLabelWrapperClass!}">
                         <label for="username" class="${properties.kcLabelClass!}"><#if !realm.loginWithEmailAllowed>${msg("username")}<#elseif !realm.registrationEmailAsUsername>${msg("usernameOrEmail")}<#else>${msg("email")}</#if></label>