check_templates: Parse '<foo/>' tags better.

If folks put something like '<br/>' in the HTML,
we would think the tag's name was "br/" instead
of "br".  I think we were assuming most folks
would write either "<br>" or <br />".

ASIDE:

We should probably have a consistent
preference among these styles:

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

I prefer the first.

tools/lib/template_parser.py

@@ -103,7 +103,11 @@ def tokenize(text: str) -> List[Token]:
                 kind = 'handlebars_singleton'
             elif looking_at_html_start():
                 s = get_html_tag(text, state.i)
-                tag_parts = s[1:-1].split()
+                if s.endswith('/>'):
+                    end_offset = -2
+                else:
+                    end_offset = -1
+                tag_parts = s[1:end_offset].split()
 
                 if not tag_parts:
                     raise TemplateParserException("Tag name missing")