Organise intro to members page

This reorganises the introduction content on member pages.

* Merges the offices, attendance and historic membership into an introduction paragraph.
* Moves rebellion figure into the header
* compacts the data range helper into the attendance sentence
* Reduces the salience of the single external link
* changes the title of the policy comparison to match the front page question, 'how do they vote?' ( relates to #740)
* overall compacts the intro (a step toward #714 in correlation with #715 )

Next iterations should do a better job of referencing the attendance and rebellion helper text, which is still messy here, a tooltip might be better.

After:
![screen shot 2014-10-14 at 8 22 40 pm](https://cloud.githubusercontent.com/assets/1239550/4626657/bc7dbe90-5383-11e4-9c0c-770f5af0a9f8.png)
![screen shot 2014-10-14 at 8 20 31 pm](https://cloud.githubusercontent.com/assets/1239550/4626640/8545f99c-5383-11e4-9f06-4df6c6226140.png)

Before:
![screen shot 2014-10-14 at 8 22 54 pm](https://cloud.githubusercontent.com/assets/1239550/4626669/d2f9b96c-5383-11e4-841d-bf3f377be88d.png)
![screen shot 2014-10-14 at 8 20 17 pm](https://cloud.githubusercontent.com/assets/1239550/4626641/88559188-5383-11e4-9570-abde42954c61.png)