This commit ensures that the Attribution, Jobs, and Team pages all
share a uniform structure to match those of other pages. This will
simplify styling and should ensure greater confidence when modifying
portico landing-page styles.
The one CSS modification here, for the jobs page, maintains the space
at the top of the "How we work" section.
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>