Add a 'hidden' field to the Person model

This field has been introduced to reduce admin burden for Mzalendo:
county representatives, for example, aren't a focus of the site, but
because they have public pages on the site, they very frequently
request updates or corrections.

We want to keep these representatives in the database, since it's useful
data that they were aspirant or elected representatives, particurly if
they then go on to be an MP, for example.  So, a reasonable solution
seems to be to make it possible to mark certain people as "hidden" so
that their public person pages no longer are shown.
Return 404s for any person page for a person that's marked hidden

Superusers can still see the person pages, but with a message at
the top saying that this page would be hidden to normal users.
Fix whitespace errors and non-PEP8 separation
Add a 'maybehidden' custom template tag

This template tag will add an 'a' tag that links to the person around
the enclosed content, so long as that person is not hidden - the link
will always be present for superusers, though.
Use maybehidden to suppress generation of links to hidden people

Having lots of broken links on the site would be bad for our ranking on
Google and confusing to users, so this suppresses any links to people
who they wouldn't be able to see the people pages for.

I haven't added these to the 'featured people' pages since presumably
you wouldn't feature someone who is hidden.

Note that the number of templates that had to be changed does suggest
that there's a lot of redundancy in how people are displayed in
templates, which would be worth looking at in the future.
Remove some trailing whitespace
Exclude hidden people from the search autocomplete

Note that this will require you to reindex all the people in
Elasticsearch.
Exclude hidden people from search results

The previous commit added the hidden attribute to the PersonIndex, so
you will need to rebuild the Elasticsearch index after that in order for
this to work.
Merge branch 'make-people-hideable'