Change metadata structure and display 

Move from lots of discrete bits of metadata to a more unified approach
with only a 'part of' or 'from' metadata.

Remove the old document_metadata method in favour of two helpers one for
part of and one for from. They both return an array of links which can
then be output in either the top or bottom of the page.

Moved the operational_field and location metadata items to the extra
metadata partial. They don't make sense in either part of or from and
are only used by one format.

New style looks like this:

![screenshot](http://f.cl.ly/items/413a0a3C19410a03332h/Screen%20Shot%202014-06-02%20at%2013.54.10.png)

https://www.pivotaltracker.com/story/show/70352364