Show hot topics and recently added votes on MP page

A two-column display of "hot topics" and "recent votes" as defined in [Policies.php](https://github.com/mysociety/theyworkforyou/blob/551-voting-summary/classes/Policies.php#L101)

![summary](https://cloud.githubusercontent.com/assets/739624/3454850/4df41062-01d7-11e4-8424-c43c0c865273.png)

![votes](https://cloud.githubusercontent.com/assets/739624/3454851/51e9400c-01d7-11e4-8488-c0d1865abfed.png)

This pull request also includes some refactoring of the vote / "dream_details" button which links each vote description to PublicWhip.

Right now, votes are shown *without* the little green / red / orange dot to signify for / against / mixed. We'll need **developer help** to pass the right variables through to the template, at which point the dots can be revealed by adding a class of `vote-description__{for,against,mix}` to each `<li>` as applicable.

Commit https://github.com/mysociety/theyworkforyou/commit/3e3caf3bd3762183d16987209754091719928d4c replaces the "external link" icons on the main MP profile page with little iOS-esque arrows. I'm not convinced it works as well. I'll revisit later.