Add the dashboard title near the submit button

Dashboard editors want to be able to see the name of the dashboard
that they're editing near the submit button, to act as a check
that they're doing the right thing.

This is helpful if you have a lot of tabs open, for example.
Merge pull request #74 from alphagov/add-dashboard-title-to-form

Add the dashboard title near the submit button
Add a link to preview the dashboard

After editing the dashboard, it is incredibly common for people
to want to view their changes. We should not make them have to
copy and paste a slug to do that.
Merge pull request #76 from alphagov/add-link-to-dashboard

Add a link to preview the dashboard