UI: add padding to sides of “favourites” featured module

The “Jetpack team favourites” module on the main Jetpack page would appear squished against the sides of the window due to a lack of padding. I added `1em` of padding to either side to tidy it up a bit.

Previously, this module looked like this:

![screen shot 2014-06-30 at 1 41 33 pm](https://cloud.githubusercontent.com/assets/343884/3433289/f5a46164-007d-11e4-8264-2994f887986b.png)

After this patch, the module looks like this:

![screen shot 2014-06-30 at 1 42 02 pm](https://cloud.githubusercontent.com/assets/343884/3433300/03109d18-007e-11e4-9ed3-f0faa2423d75.png)

**Note:** I regenerated the CSS twice. The first commit has the processed CSS after being run through the process without any changes of my own. The changes to the CSS files there are a result of autoprefixer, I believe. The second commit has the padding changes relevant to this PR.

Looking at the changes made by autoprefixer, it looks like a reinterpretation of `-webkit-background-size`. As far as I can tell, that change doesn’t affect any of Jetpack’s UI, but a reading of the spec suggests that the behaviour implemented by this would set *both* the `width` and the `height` of the element to `100%`, as opposed to how it’s written in Sass which is only setting the `width` to `100%`. As I said, no breaks as far as I can tell but maybe someone should look into that. I can rebase that regeneration out of this PR if that seems like a good idea.