Change username input type to "text"

<input type="username"> isn't allowed according to the spec and causes problems with autofill in Firefox.

share/jupyter/hub/templates/login.html

@@ -35,7 +35,7 @@
     <label for="username_input">Username:</label>
     <input
       id="username_input"
-      type="username"
+      type="text"
       autocapitalize="off"
       autocorrect="off"
       class="form-control"