Add expand and collapse links to parts

Quickly expand or collapse all parts of a guide or programme
Move a part's slug to beneath title

* Slug and title are often similar, it’s easier to edit or create both
when their fields are next to each other.
* Tweak the slug help copy and remove the obvious title copy
Style add and remove part actions as buttons
Use a single toggle all link

* With one link expand or collapse all parts.
Fix typo: "collapisibles".
Update the toggle text based on the expanded state

* Show collapse or expand links
* Change these links then collapsing/expanding all and when a user
manually changes one of the parts themselves
*There is a slight delay in the text updating because of the animation
which must finish before Bootstrap triggers the shown/hidden events
Merge pull request #267 from alphagov/expand-or-close-parts

Quickly expand or collapse parts in a guide or programme