show "back" link for password connectors

This way, the user who has selected, say, "Log in with Email" can make up
their mind, and select a different connector instead.

However, if there's only one connector set up, none of this makes sense -- and
the link will thus not be displayed.

Signed-off-by: Stephan Renatus <srenatus@chef.io>

web/templates/password.html

@@ -25,6 +25,11 @@
     <button tabindex="3" id="submit-login" type="submit" class="dex-btn theme-btn--primary">Login</button>
 
   </form>
+  {{ if .BackLink }}
+  <div class="theme-link-back">
+    <a class="dex-subtle-text" href="javascript:history.back()">Select another login method.</a>
+  </div>
+  {{ end }}
 </div>
 
 {{ template "footer.html" . }}