Remove need to generate json files for module stubs

Instead generate the module stubs at request time from the parent dashboard config
Swap fs sync methods for async versions

To avoid potentially blocking file IO.
SORN start page on GOV.UK has changed

Avoid an unnecessary redirect.
Merge pull request #534 from alphagov/update-sorn-start-page-url

SORN start page on GOV.UK has changed
Merge pull request #533 from alphagov/enhancement/remove-module-stubs

Enhancement/remove module stubs
Include https://github.com/alphagov/cheapseats/pull/71

To allow build to output stdout/stderr from server startup and be able to debug when it fails.
Amend tagline on content dashboards.

Story: https://www.pivotaltracker.com/story/show/71926036
Merge pull request #538 from alphagov/chore/update-tagline-for-content-dashboards

Amend tagline on content dashboards.
Merge pull request #537 from alphagov/enhancement/cheapseats-debug

Include https://github.com/alphagov/cheapseats/pull/71
Switch the category of visitors module to website

This module had previously been abusing the dataType field returned by
backdrop. The dataType for all of the records should be identical as the
data-set should have only one dataType. This commit will switch this
graph to using the new field website which should allow correct
switching on the data.
Merge pull request #539 from alphagov/change-visitors-category

Switch the category of visitors module to website
Correctly bind function scope in view formatter test
Move threshold for X.Xk format to 10,000
Format 1,000,000 and 1,000,000,000 to 1m and 1b respectively
Remove currency suffix code

It's never used because the only currency we support is GBP
Currency formatter always returns pence for values < 10
Add some extra test coverage for number formatters
Updated test for sigfigs behaviour

Sigfigs only applies to decimals, not to whole numbers. It's a slight bastardisation of the concept of sigfigs, but it works for our purposes.
Handle decimal places for small values in currency formatter.

Values < 10 should have 2 dps by default, and >= 10 should always be integers
Ensure percentage formatter returns unformatted value for non-numbers