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
Refactor formatters from view prototype to use as uch of standard formatter lib as possible
Remove tests for numbers < 10000 formatting as X.Xk
Completely remove `View#formatDuration`

Use standard formatters instead
Change billion suffix in tests to "bn"
Make zero always return zero
Assorted test fixes
Move numberListFormatter method into main formatters script
Completely remove formatPercentage function

Use standard formatters lib instead
Fix issue with decimal places option being overwritten in numberListFormatter

Passing 0 to currency formatter sets dps option to 2, which is then re-used for subsequent formattings. Cloning the object means a fresh set of options is used each time, and value specific settings are not overwritten.