Added [alt] attribute to provider image elements (#177)

* Added [alt] attribute to provider image elements

* Changed image element [alt] attribute to empty string

Author: TarikHuber

soy/elements.soy

@@ -124,6 +124,7 @@
     <span class="firebaseui-idp-icon-wrapper">
       <img
           class="firebaseui-idp-icon"
+          alt=""
           src="{call .idpLogo_ data="all" /}">
     </span>
     {if $providerId == 'password'}