Fix functional tests for history

This moves two tests for the show action into the correct part of this 
functional test file, and changes the history test to request the 
history action now instead.
Allow the slug to be changed for a draft artefact

The slug can be changed for an artefact whilst it is in the draft state, 
so display the slug field for draft artefacts.
Fix the Cucumber test for changing an artefact

Previously, these tests "changed" the title of the artefact, even though 
the field was disabled. Now that the title field has been removed 
outright, this test breaks.

I've changed this to edit the Need ID instead, as it's the only other 
text field that's consistently on the page.