Replace old image button with a simple button

- add fixes for mobile and medium screens
- new style selectors
- button background-color (tablets / medium screens)
- small adjustments
- remove unused image file

Signed-off-by: RD WebDesign <github@rdwebdesign.com.br>

scripts/pi-hole/php/header.php

@@ -313,10 +313,10 @@ if($auth) {
                             </li>
                             <!-- Menu Footer -->
                             <li class="user-footer">
-                                <!-- PayPal -->
+                                <!-- Donate Button -->
                                 <div class="text-center">
-                                    <a href="https://pi-hole.net/donate/" rel="noopener" target="_blank">
-                                        <img src="img/donate.gif" alt="Donate" width="147" height="47">
+                                    <a class="btn btn-primary btn-lg donate" href="https://pi-hole.net/donate/" rel="noopener" target="_blank">
+                                        <i class="fas fa-fw menu-icon fa-donate"></i> Donate
                                     </a>
                                 </div>
                             </li>