Add admin-settable MOTD, rewrite existing warning system to use it

templates/default/index.html

@@ -1,5 +1,16 @@
 <script src="assets/chartjs/chart.js"></script>
 
+{{if .MOTD}}
+<article class="card {{.MOTD.Style}}">
+    <header>
+        <h3>{{.MOTD.Title}}</h3>
+    </header>
+    <footer>
+        <p>{{.MOTD.Content}}</p>
+    </footer>
+</article>
+{{end}}
+
 <h1>Green report</h1>
 
 <canvas id="report"></canvas>
@@ -7,7 +18,6 @@
     This server's energy statistics for the last eight days (current day included)
 </p>
 
-
 <div class="flex two">
     <div>
         <div class="home-cards card">