Smaller font in single stat module

This fixes rendering of modules on two columns in small resolution
devices

@maxfliri
@gtrogers
Consistent styling of single stat modules

@gtrogers, @maxfliri
Fix the tests

@maxfliri
@gtrogers
Valid markup in live visitors module

@gtrogers
@maxfliri
Merge pull request #116 from alphagov/fix-single-number-module-font-size

Single stat module improvements