Return user to summary page after clicking 'edit'

When you edit a page, saving your changes should return you to
the summary page.

The default flow is still to complete one page after another.

This commit add a URL parameter (`return_to_summary`). On the
'edit' links on the summary page it is set to `yes`.

When you view a page which has `?return_to_summary=yes` after the
URL the button will say 'Save and return to summary' rather than
'Save and continue'. Clicking the button will do what the button
says it does.
Remove legacy Boolean from method params
Removed debug gubbins
Merge pull request #129 from alphagov/save-and-return-to-summary

Return user to summary page after clicking 'edit' and saving