Adds HTML title elements to templates that extend either `base.html`,
`portico.html` or `portico_signup.html`, and that are not website
portico landing pages that will use the `PAGE_TITLE` variable to set
the HTML title element (see following commit in series).
Also, updates some templates for missing translation tags.
As a general rule, we want the title element (and page content)
translated. Exceptions that are updated in this commit are templates
used in the development environment, analytics templates that are used
by staff and templates related to Zephyr.
We fixed the main issue of this form in CVE-2020-9444, but the audit
done at that time only included links found in rendered_markdown; this
change completes our audit for links with target=_blank anywhere in
the codebase.
We're migrating to using the cleaner zulip.com domain, which involves
changing all of our links from ReadTheDocs and other places to point
to the cleaner URL.
This allows us to block use of the desktop app with insecure versions
(we simply fail to load the Zulip webapp at all, instead rendering an
error page).
For now we block only versions that are known to be both insecure and
not auto-updating, but we can easily adjust these parameters in the
future.