Add option for default values in completion numbers module

In the site activity dashboards, the feedback modules only get data pushed to backdrop when feedback is submitted, so they should contain 0 by default when no data is in backdrop.
Default site activity feedback values to 0
Add meta description for transaction pages.

This is for SEO purposes, in an effort to reduce the number of users who
click through to our transaction pages by mistake after entering generic
transaction-related search terms.

The meta description tries to make it clearer that this is a performance
dashboard with statistics - not the service itself.

Background here: https://www.pivotaltracker.com/s/projects/911874/stories/71237550
Add tests for meta description tag.
Finalise "View driving record" dashboard

- Move into correct place, set published to true
- Add description

https://www.pivotaltracker.com/story/show/71555900
completion rate for visa
Merge pull request #504 from alphagov/feature/meta-description-tag-for-seo

Add meta description for transaction pages.
Merge pull request #511 from alphagov/feature/completion-numbers-default-values

Add option for default values in completion numbers module
Merge pull request #514 from alphagov/feature/visa-completion

completion rate for visa
Merge pull request #513 from alphagov/finalise-view-driving-record

Finalise "View driving record" dashboard
Update wording of info text on driving record live usage module
Update info copy on view driving record user login module