Restrict bootstrap component visited link styles

* Only apply the styles to specified descendants
* eg fixes a situation in Transition where a drop down is nested in a
navigation pill, which caused broken hover states for visited links
Include visit styles for active links in dropdowns

* Visited links were showing in black if they were also active within a
dropdown
Match list group item visited colour with active

* Use component active colour (white) rather than the secondary active
colour (a light blue)
* Include examples of normal active links in style guide
Merge pull request #18 from alphagov/fix-nested-visited-link-colours

Restrict bootstrap component visited link styles