Add basic deadline to sections index

app/javascript/deadline_display.js

@@ -0,0 +1,27 @@
+export default function initDeadlineDisplay() {
+    document.querySelectorAll('[data-behaviour="deadline_display"]').forEach((span) => {
+        const deadline = new Date(span.dataset.deadline)
+
+        function update() {
+            const remaining = Math.floor((deadline - new Date()) / 1000)
+            const days = Math.floor(remaining / (60 * 60 * 24))
+            const hours = Math.floor((remaining % (60 * 60 * 24)) / (60 * 60))
+            const minutes = Math.floor((remaining % (60 * 60)) / 60)
+            const seconds = remaining % 60
+            let remainingText = ""
+            if (days > 0) {
+                remainingText += `${days} dagen, `
+            }
+            if (days > 0 || hours > 0) {
+                remainingText += `${hours} uren, `
+            }
+            if (days > 0 || hours > 0 || minutes > 0) {
+                remainingText += `${minutes} minuten en `
+            }
+            remainingText += `${seconds} seconden`
+            span.innerText =  remainingText
+        }
+
+        setInterval(update, 200)
+    })
+}