Move to bootstrap-sass and Formtastic 2.2

This changes the app to use bootstrap-sass and updates our version of 
Formtastic to 2.2.1.

Bootstrap is now imported in application.scss and the previous overrides 
file is updated so that our custom styles still apply.

Using @import rather than manifest require statements is preferred so 
that we can extend Bootstrap styles in our own CSS rules.
Swap out chosen files for gem-provided version

The chosen-rails gem provides a version of chosen which can be included 
using the asset pipeline. This swaps out our existing copy that lived in 
vendor/assets.
Make the typography look similar to before

Bootstrap 2.3.2 defaults the font-size to 14px, where we have previously 
used 13px. This sets 13px (and a line-height of 18px) as the default.
First pass at a new artefact edit page

This splits the artefact form into two halves, using multiple wells to 
split up sections of the form instead of one large well.

Tabs have been removed, replaced with buttons to view history or archive 
the artefact: currently stubs to be implemented in a later commit.

I've removed the Name, Slug and Description fields for existing artefacts, 
as they would be disabled by default anyway. Unfortunately this breaks 
a few tests, so I've fixed some of them here.
Make wells look less rubbish

Maybe it's just me, but I find that Bootstrap's wells look a bit 
wishy-washy. I've removed the box shadow, made the border darker and 
removed the rounded corners.
Tidy up the related items fields

- Make it clearer that related artefacts can be sorted
- Make both related artefacts and related links look similar
- Fix some of the spacing issues
Tidy up tag fields
Add a label to indicate the state of the artefact
Add separate history view and archive modal box
Make owning app text bigger, and tone down buttons
Tweak form styles a bit

Based on feedback from @markhurrell, I've removed the borders from
nested items (like related artefacts and sections), moved the
button group from the far right-hand side to next to the title, and
bodged the Bootstrap proportions so that there isn't such a big
gap between the two columns.
Split form into lots of partials

I think this makes it cleaner.
Tighten the scope for the Bootstrap span4 bodge rule
Add an external link icon to the slug

This makes it clearer that the slug is a link to the content on the 
live site.
Make the archive view a separate page

This is in preparation for changing the button group to tabs.
Use tabs instead of the button groups

Following Maslow's change to use tabs instead of button groups, I'm 
going to abandon the change to remove tabs here.
Fix a unit test for the ArtefactsHelper
First pass at a new integration test for the edit form

This asserts that many elements of the form are displayed correctly.
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.