create filter-search.js file in /static/js directory.

This file contains the code for the filtering feature in pages.html and
archive.html -- a basic search feature which works by filtering the list of
entries on the page.

It was originally in main.js but I moved it here because it was producing errors
on pages which didn't have the filer stuff on the HTML template. This change
allows each template to call it when the template actually uses it.

static/js/filter-search.js

@@ -0,0 +1,18 @@
+ window.addEventListener("load", () => {
+     // (A) GET HTML ELEMENTS
+     var filter = document.getElementById("fe-search-filter"), // search box
+         list = document.querySelectorAll("#fe-search-filter-list li"); // all list items
+     
+     // (B) ATTACH KEY UP LISTENER TO SEARCH BOX
+     filter.onkeyup = () => {
+         // (B1) GET CURRENT SEARCH TERM
+         let search = filter.value.toLowerCase();
+         
+         // (B2) LOOP THROUGH LIST ITEMS - ONLY SHOW THOSE THAT MATCH SEARCH
+         for (let i of list) {
+             let item = i.innerHTML.toLowerCase();
+             if (item.indexOf(search) == -1) { i.classList.add("hide"); }
+             else { i.classList.remove("hide"); }
+         }
+     };
+ });