Use more real estate on xl screens

Prior to this change, on a screem >1200px wide the .container was
capped at 1140px, which leaves a lot of whitespace.

This increases the cap to 1900px, more effectively using
larger screens while still preventing the text from getting so
wide as to be hard to read.

Partial fix for #109

static/src/scss/app.scss

@@ -125,6 +125,11 @@ main.container-fluid {
     padding: 0;
 }
 
+@include media-breakpoint-up(xl) {
+    .container{
+        max-width: 1900px
+    }
+}