versioning: add mike css and js files

docs/css/version-select.css

@@ -0,0 +1,5 @@
+@media only screen and (max-width:76.1875em) {
+  #version-selector {
+    padding: .6rem .8rem;
+  }
+}

docs/js/version-select.js

@@ -0,0 +1,50 @@
+window.addEventListener("DOMContentLoaded", function() {
+  // This is a bit hacky. Figure out the base URL from a known CSS file the
+  // template refers to...
+  var ex = new RegExp("/?assets/fonts/material-icons.css$");
+  var sheet = document.querySelector('link[href$="material-icons.css"]');
+
+  var REL_BASE_URL = sheet.getAttribute("href").replace(ex, "");
+  var ABS_BASE_URL = sheet.href.replace(ex, "");
+  var CURRENT_VERSION = ABS_BASE_URL.split("/").pop();
+
+  function makeSelect(options, selected) {
+    var select = document.createElement("select");
+    select.classList.add("form-control");
+
+    options.forEach(function(i) {
+      var option = new Option(i.text, i.value, undefined,
+                              i.value === selected);
+      select.add(option);
+    });
+
+    return select;
+  }
+
+  var xhr = new XMLHttpRequest();
+  xhr.open("GET", REL_BASE_URL + "/../versions.json");
+  xhr.onload = function() {
+    var versions = JSON.parse(this.responseText);
+
+    var realVersion = versions.find(function(i) {
+      return i.version === CURRENT_VERSION ||
+             i.aliases.includes(CURRENT_VERSION);
+    }).version;
+
+    var select = makeSelect(versions.map(function(i) {
+      return {text: i.title, value: i.version};
+    }), realVersion);
+    select.addEventListener("change", function(event) {
+      window.location.href = REL_BASE_URL + "/../" + this.value;
+    });
+
+    var container = document.createElement("div");
+    container.id = "version-selector";
+    container.className = "md-nav__item";
+    container.appendChild(select);
+
+    var sidebar = document.querySelector(".md-nav--primary > .md-nav__list");
+    sidebar.parentNode.insertBefore(container, sidebar);
+  };
+  xhr.send();
+});