templates: Use <hr> and <br> consistently.

We now prevent these variations:

    * <hr/>
    * <hr />
    * <br/>
    * <br />

We could enforce similar consistency for other void
tags, if we wished, but these two are particularly
prevalent.

zerver/lib/bugdown/api_arguments_table_generator.py

@@ -98,7 +98,7 @@ class APIArgumentsTablePreprocessor(Preprocessor):
         <span class="api-argument-example-label">Example</span>: <code>{example}</code>
     </div>
     <div class="api-description">{description}</div>
-    <hr />
+    <hr>
 </div>"""
 
         md_engine = markdown.Markdown(extensions=[])