[#642] Missing state: active link

[planktonic]

Fixes #642

Changes

• Adds variables for specifying appearance of :active links
• Applies those properties to the active state of <a> elements and .a-links

Checks

Delete if not applicable:

• [x] Fixtures in test/scss/ have been updated
• [x] Your changes have been added to the unreleased section of CHANGELOG.md