Copy changes for party speeches instances.

Correct end date for labour speeches to 2013.
Better paragraph about the conservative speeches disappearing.
Simplify text about republishing the Labour speeches.
Link to the labour archive.
Make Speeches in the navigation point to /speeches.

In the party speech instances we want the Speeches navigation to
point to /speeches even if there is an about page.

A better fix would be to have the link always point to /speeches
unless overridden.