Avoid dependency on lodash

This commit re-implements `_.groupBy`. The call to `_.sortBy` was
unnecessary and was thus removed entirely.

This should simplify migration to newer releases of Hexo that no longer
include lodash.

themes/starter/layout/tag.ejs

@@ -17,7 +17,7 @@
   </div>
   <div class="one-tag-list">
     <span class="tag__text"><%= title %></span>
-    <% const groupObj = _.groupBy(page.posts.toArray(), function(p) {return -p.date.format('YYYY')}) %>
+    <% const groupObj = groupBy(page.posts.toArray(), function(p) {return -p.date.format('YYYY')}) %>
     <% for (let year in groupObj){ %>
       <h3 class="tags__year"><%- -year %></h3>
       <ul class="tags__list">

themes/starter/layout/tags.ejs

@@ -10,11 +10,11 @@ layout: layout
     <% }) %>
   </div>
   <div class="one-tag-list">
-    <% const groupObj = _.groupBy(site.posts.sort('date', -1).toArray(), function(p){return -p.date.format('YYYY')}) %>
+    <% const groupObj = groupBy(site.posts.sort('date', -1).toArray(), function(p){return -p.date.format('YYYY')}) %>
     <% for (let year in groupObj){ %>
     <h3 class="tags__year"><%- -year %></h3>
     <ul class="tags__list">
-      <%- partial('_partial/taglist', {posts: _.sortBy(groupObj[year], 'date').reverse()}) %>
+      <%- partial('_partial/taglist', {posts: groupObj[year]}) %>
     </ul>
     <% } %>
   </div>

themes/starter/scripts/hexo-helpers.js

@@ -5,6 +5,7 @@ hexo.extend.helper.register('get_langs', getLangs);
 hexo.extend.helper.register('switch_lang', switchLang);
 hexo.extend.helper.register('getPath', getPath);
 hexo.extend.helper.register('stripHTTPS', stripHTTPS);
+hexo.extend.helper.register('groupBy', groupBy);
 
 function isRoot() {
   const lang = this.page.lang;
@@ -36,3 +37,15 @@ function getPath() {
 function stripHTTPS(x) {
   return x.split('://')[1];
 }
+
+function groupBy(arr, fn) {
+  let result = {};
+  for (const element of arr) {
+     const key = fn(element);
+     if (!(key in result)) {
+       result[key] = new Array();
+     }
+     result[key].push(element);
+  }
+  return result;
+}