Make HTML Beautifier fail on nesting errors

This way we're also checking mistakes like closing tags that don't match
their opening element, which we detected by manually running HTML
Beautifier with the `-e` option, and fixed two commits ago.

Note there was a false positive in the mailer layout. We don't know the
cause. Maybe closing the ERB tag right before the HTML opening tag and
the lack of other attributes on the tag made HTML Beautifier think the
tag wasn't correctly open, but on the other hand, we have the exact same
line in other layouts where HTML Beautifier works fine. We're fixing it
by adding an HTML id attribute to the element.

.github/workflows/linters.yml

@@ -22,7 +22,7 @@ jobs:
           - name: Markdownlint
             run: bundle exec mdl *.md docs/
           - name: HTML Beautifier
-            run: find app/ -name "*.html.erb" -exec bundle exec htmlbeautifier --lint-only --keep-blank-lines 1 {} +
+            run: find app/ -name "*.html.erb" -exec bundle exec htmlbeautifier --lint-only --keep-blank-lines 1 -e {} +
     name: ${{ matrix.name }}
     steps:
       - name: Checkout code