Enterprise sign in page UI improvements.

ENT-448

lms/static/sass/views/_login-register.scss

@@ -681,7 +681,11 @@
         height: 100px;
     }
 
-    h2 {
+    p {
+        &:first-of-type {
+            margin-top: 20px;
+        }
+        
         font-size: 16px;
         line-height: 1.5;
         color: $gray-d2;

lms/templates/student_account/enterprise_sidebar.html

@@ -3,11 +3,11 @@
         % if enterprise_logo_url:
             <img src="${enterprise_logo_url}" alt="${enterprise_name}" class="enterprise-logo">
         % endif
-        <h2>
-            ${enterprise_branded_welcome_string}
-        </h2>
-        <h2>
+        <p>
             ${platform_welcome_string}
-        </h2>
+        </p>
+        <p>
+            ${enterprise_branded_welcome_string}
+        </p>
     </div>
 </div>