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.

.github/workflows/linters.yml

@@ -21,6 +21,8 @@ jobs:
             run: npx stylelint app/assets/stylesheets
           - 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 {} +
     name: ${{ matrix.name }}
     steps:
       - name: Checkout code