Add support for hiding people on the site

There are many people in the Pombola people database who aren't
the focus of the site (for example old aspirants for ward representative
positions) but who generate a lot of support requests for updates to
their profiles. @JessMusila has requested that these people should
either be removed or hidden from the site to reduce this burden on the
administrators of the site.  This pull request adds support for hiding
people in Pombola - this means that they won't have person pages
any more or appear in search results.  It also makes sure that in cases
where these people might otherwise be linked to (e.g. from
organisation or place pages) those links are suppressed.