Add a test to reproduce the local transaction bug.

This appears to happen whenever there's an error editing a local
transaction, because it tries to render through the
`local_transactions/show` template, which got missed in the template
refactoring from <https://github.com/alphagov/publisher/pull/256>.
Fix local-transaction validation error path.

There's a difference in the way template paths are resolved between
doing a `render :action` and `render :template`.  The latter was finding
the spurious local_transactions show template that had been left behind
from the refactoring in #256.

This also cleans up the redundant template (it's not used, and can't
work any more as it references partials that no longer exist.)
Merge pull request #259 from alphagov/fix-local-transaction-editing

Fix local-transaction template error on validation error.