Use correct variables when styling links

Overriding frontend toolkit in colours.scss. This is so we can
reuse this for link-ish things elsewhere.
Fix styling of page history toggles
Underline focused links to match hover style
Reduce default_cache_max_age to 15 minutes

Also made configurable via initializer

https://www.pivotaltracker.com/story/show/54915576
Fix tests to match new cache time
Merge pull request #739 from alphagov/fix-styles-on-history-toggles

Fix styles on history toggles
Merge pull request #748 from alphagov/54915576-reduce-default-cache-max-age-to-15-mins

Reduce default_cache_max_age to 15 minutes