Add woff2 fonts

This should close #1233 and add 4 weights of the font Lato.

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

static/themes/default/css/plinth.css

@@ -1,12 +1,64 @@
 @font-face {
-    font-family: LatoRegular;
+    font-family: 'Lato';
-    src: url('../lato/Lato-Regular.ttf');
+    src: url('../lato/Lato-Italic.woff2') format('woff2');
+    font-style: italic;
+    font-weight: 400;
 }
 
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-Regular.woff2') format('woff2');
+    font-style: normal;
+    font-weight: 400;
+}
+
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-Semibold.woff2') format('woff2');
+    font-style: normal;
+    font-weight: 600;
+}
+
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-SemiboldItalic.woff2') format('woff2');
+    font-style: italic;
+    font-weight: 600;
+}
+
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-Bold.woff2') format('woff2');
+    font-style: normal;
+    font-weight: 700;
+}
+
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-BoldItalic.woff2') format('woff2');
+    font-style: italic;
+    font-weight: 700;
+}
+
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-Heavy.woff2') format('woff2');
+    font-style: normal;
+    font-weight: 800;
+}
+
+@font-face {
+    font-family: 'Lato';
+    src: url('../lato/Lato-HeavyItalic.woff2') format('woff2');
+    font-style: italic;
+    font-weight: 800;
+}
+
+
 body {
     padding-top: 6em;
     position: relative;
-    font-family: LatoRegular, sans-serif;
+    font-family: Lato, sans-serif;
     font-size: 15px;
     background-repeat: no-repeat;
     background-position: 22.5px 75px;