Merge branch 'master', remote-tracking branch 'origin'
[#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.
[#419] Fix link in updates edge case

When an updates only consisted of a link that would
not get urlized. We have to do some escape dancing to
make sure urls are urlized by Django & that markdown
works properly. And at the same time be safe and not
allow things like <script>alert('unsafe');</script>
to be evaluated.
#456 adding testing issue to Release Notes.
Merge pull request #461 from akvo/release/candidate

Merged release 2.3.4.
Merge pull request #492 from akvo/master

Update develop with missing commits from master
Merging to resolve the issue.