Use HTML beautifier to indent ERB files

We had inconsistent indentation in many places. Now we're fixing them
and adding a linter to our CI so we don't accidentally introduce
inconsistent indentations again.

app/views/admin/stats/show.html.erb

@@ -56,7 +56,9 @@
         <div class="small-12 medium-3 column">
           <p class="featured">
             <%= t "admin.stats.show.summary.proposal_votes" %> <br>
-            <span class="number"><%= number_with_delimiter(@proposal_votes) %> <br></span>
+            <span class="number">
+              <%= number_with_delimiter(@proposal_votes) %> <br>
+            </span>
           </p>
           <p>
             <%= t "admin.stats.show.summary.debate_votes" %> <br>