Add buttons to go through posts

Closes #8

src/site/html/static/js/post_series.js

@@ -0,0 +1,56 @@
+import {controller} from "./pagination.js"
+
+class Timeline {
+	constructor() {
+		this.shortcodes = []
+		/** @type {Map<string, TimelineEntry>} */
+		this.entries = new Map()
+		controller.addActivatedCallback(() => this.update())
+		this.update()
+	}
+
+	update() {
+		this.shortcodes = []
+		document.querySelectorAll("#timeline .sized-link").forEach(element => {
+			const shortcode = element.getAttribute("data-shortcode")
+			this.shortcodes.push(shortcode)
+			this.entries.set(shortcode, new TimelineEntry(this, shortcode))
+		})
+		console.log(this.shortcodes)
+	}
+
+	fetch() {
+		return controller.activate()
+	}
+}
+
+/**
+ * @param {Timeline} timeline
+ * @param {string} shortcode
+ */
+class TimelineEntry {
+	constructor(timeline, shortcode) {
+		this.timeline = timeline
+		this.shortcode = shortcode
+	}
+
+	getNextShortcode() {
+		return this.timeline.shortcodes[this.timeline.shortcodes.indexOf(this.shortcode)+1]
+	}
+
+	getPreviousShortcode() {
+		return this.timeline.shortcodes[this.timeline.shortcodes.indexOf(this.shortcode)-1]
+	}
+
+	isFirstEntry() {
+		return this.timeline.shortcodes.indexOf(this.shortcode) === 0
+	}
+
+	isLastEntry() {
+		return this.timeline.shortcodes.indexOf(this.shortcode) === this.timeline.shortcodes.length-1
+	}
+}
+
+const timeline = new Timeline()
+
+export {timeline}