Add some feedback when performing some actions (#552)

* Add a way to display flash messages

* Make the flash messages look nice

* Add actual feedback messages

* cargo fmt

* Move flash messages to PlumeRocket

And add trait to convert PlumeRocket to BaseContext

* Remove useless lifetime

templates/base.rs.html

@@ -69,6 +69,11 @@
                 </nav>
             </div>
         </header>
+        <div class="messages">
+            @if let Some(ref message) = ctx.3 {
+                <p class="flash-message @message.0">@message.1</p>
+            }
+        </div>
         <main>
             @:content()
         </main>