Replace smart_truncate with Gem

In d86e1a2, we introduced post previews on the homepage (so you get a
preview on the homepage, but have to visit the post itself to continue
reading). To do so, we added a Jekyll "plugin" which is actually just a
single ruby file -- `smart_truncate.rb`. (This plugin is open source and
came from https://yingtongli.me/git/smart_truncate/tree/smart_truncate.rb.)

That plugin works, but I was disappointed there wasn't an easier way to
do this (a built-in Jekyll feature or a plugin distributed as a gem).
So, I built my own
[jekyll_html_truncatewords](https://github.com/mkasberg/jekyll_html_trunctewords)
plugin. This plugin is distributed as a gem and has better unit test
coverage than the `smart_truncate.rb` file (which had no apparent unit
tests), so I feel better about using it. My intent is to make this a
reliable and well-maintained Jekyll plugin for use here and for anyone
else who wants to use it.

In this commit, I remove the `_plugins/smart_truncate.rb` file and
replace it with my `jekyll_html_truncatewords` gem. The functionality is
identical, and I spot-checked several rendered HTML pages which look
identical with either plugin. The end result is that we don't have to
maintain a copy of that `smart_truncate.rb` file and instead we can
update the gem like we would for any other Jekyll gem/plugin.

Author: mkasberg

Gemfile

@@ -3,4 +3,7 @@ source 'https://rubygems.org/'
 gem 'jekyll', '~> 4.1'
 gem 'jekyll-redirect-from', '~> 0.16'
 gem 'jekyll-paginate', '~> 1.1'
-gem 'nokogiri', '~> 1.10'
+
+group :jekyll_plugins do
+    gem 'jekyll_html_truncatewords', '~> 0.1.2'
+end

Gemfile.lock

@@ -36,6 +36,9 @@ GEM
       sassc (> 2.0.1, < 3.0)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
+    jekyll_html_truncatewords (0.1.2)
+      liquid
+      nokogiri
     kramdown (2.3.0)
       rexml
     kramdown-parser-gfm (1.1.0)
@@ -45,12 +48,14 @@ GEM
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.4.0)
-    mini_portile2 (2.4.0)
-    nokogiri (1.10.10)
-      mini_portile2 (~> 2.4.0)
+    mini_portile2 (2.5.0)
+    nokogiri (1.11.1)
+      mini_portile2 (~> 2.5.0)
+      racc (~> 1.4)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.6)
+    racc (1.5.2)
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
@@ -70,7 +75,7 @@ DEPENDENCIES
   jekyll (~> 4.1)
   jekyll-paginate (~> 1.1)
   jekyll-redirect-from (~> 0.16)
-  nokogiri (~> 1.10)
+  jekyll_html_truncatewords (~> 0.1.2)
 
 BUNDLED WITH
    2.1.4

_includes/post_preview.html

@@ -7,7 +7,7 @@ <h2>
 </h2>
 
 <div class="post-content" itemprop="articleBody">
-  {{ post.content | markdownify | smart_truncate: site.post_preview_words }}
+  {{ post.content | markdownify | html_truncatewords: site.post_preview_words }}
   <a href="{{ post.url | prepend: site.baseurl }}">Continue reading</a>
 </div>