Removes the default title element of "Zulip" from `base.html` and
the default meta-description sentence from `meta_tags.html`. Also
removes default open graph metadata.
For portico templates that would would fail tests, set both
`PAGE_TITLE` and `PAGE_DESCRIPTION` variables with appropriate
content.
Co-authored-by: Alya Abbott <alya@zulip.com>
Aman Agrawal
Alex Vandiver
Lauryn Menard