Allow adding source urls to a person

This adds an extra tab to the person pages which allows adding source
urls to the record to indicate where the information for them comes
from.
Allow adding source urls to organizations
Make each source into a link to the source

Since a source is a url it makes sense to actually link to the source
when displaying it to the user.
Sources are arrays of link objects, not links

Each link object within the sources array can have a url and note
attached to it. Previously it had been mistakenly implemented as an
array of urls.
Update popit-api to latest master
display translated text according to http accept header

use the http accept header to work out which language the user is
requesting and display it in that language if available otherwise fall
back to the default
save translated items using http accept

uses http accept to determine the language and then updates/created
translation objects for the saved data
Add a language switcher

At the moment we're limiting this to one of two languages so this also
involves adding an alternative language setting to the instance.

Also, refactor out the code to get the different languages to it's own
method which now checks the session for language and overrides the http
accept set language with that.

Plus template for language switcher, and inclusion thereof in
templates