[fix bug 1016500] Manifesto page CSS style updates and fixes

**Not yet ready for merge**

Still waiting on final confirmation to these changes, but it can at least be ready for code review. This PR includes mainly CSS updates to link styles & hover/focus states, as requested by Lee & Holly. There are also a few other fixes/updates:

* Principal block links now display a shaded border for hover/focus states.
* "See more" links for principal blocks are now only visible on smaller viewport sizes, where hover interactions are less likely.
* Updated a couple of links in principal 8 (used l10n tag for the string change).
* Link styles have also been updated for "Learn more about" and "Tweet this" on the main page, as well as "Learn more" links in the modals for consistency.
* Added some missing CSS vendor prefixes.
* Moved `matchMedia.addListener.js` to a conditional comment, as it throws an error in IE8.