This makes the /help/ sidebar more discoverable at windows less
than 1000px in width because it makes it stick out a bit when it
is closed with the hamburger menu at the top.
Fixes: #6038.
Apparently, the filters implementation was doing some sort of strange
caching, where you would need to restart the server in order to
refresh for changes to the markdown content.
We fix this by switching to just calling the render_markdown_path
function from Jinja2.
Fixes#5974.
This redesigns the /help/ page sets to be a single page app that uses
history.pushState to work the same as the old app.
The big new feature is that now we have the index in a nicely designed
left sidebar.
This styles inline images, the markdown page as a whole, and the
notification buttons to be CSS rather than images.
[Tweaked by tabbott to temporarily remove the Yantramanov font, since
we should be committing that to the repo so Zulip works correctly without
Internet access]