[#381] Add TimestampsAdminDisplayMixin

TimestampsAdminDisplayMixin is used in the admin to display the
timestamp fields, created_at and last_modified_at, if they hold any data.

The fields are displayed last in the first fieldset of the form,
using get_fieldsets() to hide/show them.
[#381 #458] Display timestamps in the admin for relevant models

Using the TimestampsAdminDisplayMixin add the display of timestamp
fields to Organisation, Project, ProjectUpdate and PartnerSite model
admins.

Change where the fields are displayed to kill bug #458 that happened
when the timestamp fields had their own fieldset that was only shown
when there was any timestamp data.
[#458] Cleanup of organisation/change_form.html

The organisation/change_form.html had some code still from when it
showed internal organisation IDs thata was removed.