This restructures the styling for the Zulip settings and
administration pages to minimize use of Bootstrap and use a consistent
styling library for similar elements.
While it is basically a wash in terms of the page's visuals, it will
make our life a lot easier for future work on improving the settings
pages section of the site.
The check-handlebars-templates script now looks at most of our
back end templates to try and find imbalanced tags. This commit
fixes a bunch of the existing templates.
(imported from commit fad4a5d85d68160370dd588b41d6f125f64d198f)