fix(mail(js)): hide sign and encrypt options if not usable

When composing a message, don't offer to sign nor encrypt the message
if user has not uploaded a certificate or if S/MIME is disabled.

UI/WebServerResources/js/Mailer/Account.service.js

@@ -392,6 +392,16 @@
     return identity.textSignature;
   };
 
+  /**
+   * @function $hasCertificate
+   * @memberof Account.prototype
+   * @desc Return true if the user has a S/MIME certificate for this account
+   * @returns a boolean value
+   */
+  Account.prototype.$hasCertificate = function() {
+    return this.security && this.security.hasCertificate;
+  };
+
   /**
    * @function $certificate
    * @memberof Account.prototype
@@ -401,7 +411,7 @@
   Account.prototype.$certificate = function() {
     var _this = this;
 
-    if (this.security && this.security.hasCertificate) {
+    if (this.$hasCertificate()) {
       if (this.$$certificate)
         return Account.$q.when(this.$$certificate);
       else {