Refactor most popular video component to match new facia styles

This is remains a unique pattern to video pages. ie. as it's in the RH column at desktop we don't have this as a pattern in new facia containers. So have just updated the styling to match the new container styles as per @Pearson82 guidelines.

- Font weight of headlines to 500.
- Video icon to yellow
- Remove yellow border-top at cell level.
- Change border above 'popular videos' to `$c-neutral2`
- At mobile/tablet breakpoints there was excessive amount of space above and below the container.
- Currently we are missing the border above the container at mobile. Can we keep this at mobile as per other breakpoints.

## Before
![google chrome](https://cloud.githubusercontent.com/assets/80089/4800700/dd15f3d4-5e28-11e4-8673-01074d466fc6.png)

## After
### Mobile
![google chrome](https://cloud.githubusercontent.com/assets/80089/4800701/e13e087a-5e28-11e4-88f4-b80470987f23.png)
 
### Tablet
![google chrome](https://cloud.githubusercontent.com/assets/80089/4800705/ea82dfe6-5e28-11e4-90c5-ffc51945f11a.png)

### Desktop
![google chrome](https://cloud.githubusercontent.com/assets/80089/4800725/21ac81b6-5e29-11e4-964c-44b0982d2f75.png)