Remove duplicate call to dates function
Ensure tests use same collapse_key as code.

Also reuse options across tests as much as possible, and add basic data
for multi-page sections that share page title.
Flatten data and add _ids: push to Backdrop.

This may change, since it hasn't been tested against a real Backdrop
deployment yet, but checking in here for clarify.

Also update tests.
Use shortest common URL as canonical URL for pages with same title.

If we have a series of transaction pages such as:

* `/student-loans`
* `/student-loans/start-application`
* `/student-loands/your-details`

etc, usually these pages all have the same title.

So we want to ensure that we're using the first URL as the canonical URL for
this entry, so that we can link to the canonical page in the displayed data.

Also update test data to ensure this is happening.
Use path as unique key, and encode in base 64.

Use paths rather than titles as keys because Backdrop rejects keys with
whitespace in. Encode in base 64 to ensure Backdrop will be happy with
the keys we give it.

Update tests.