Add tabbed completion_rate graph for "Proportion of SMEs" module

Pivotal story: https://www.pivotaltracker.com/s/projects/911874/stories/62377784

Our first ever tabbed completion_rate graph!

Changes made: 

* Finalise Backdrop stub data, responses, and Stagecraft stubs, following discussions with Data In. 
* Add parse method to completion_rate collection to handle nested data: we need this because Backdrop always returned nested data whenever a query is grouped, but our existing completion_rate graphs expect flat data. 
* Add tabs & amend tabs view to update valueAttribute option on tab click.
* Amend completion collection to ensure valueAttribute option is always up-to-date in the completion rate calculations. 
* Update tests for tabs and completion_rate module. 

Also a few general changes affecting the G-Cloud dashboard, included in this branch: 

* Remove currency options from count view of "division of procurement" graph: https://github.com/alphagov/spotlight/commit/3f5b0034c7ea1d76ff4414baf9f9e433d8e0e0de
* Alter tab margin and padding (NB affects all tabbed graphs on Spotlight - Michael has approved): https://github.com/alphagov/spotlight/commit/7c5a205c1d37282688258d6fe95c72ddf2b4f124
* Change wording for "Unknown" categories: https://github.com/alphagov/spotlight/commit/6cbf06c42f7b21aed3bc55eb03f909c38bc91568