Add some basic mobile / tablet styles to the Momentum homepage

[lodyb]

This PR adds some basic styles to the homepage.

I think it's in a decent state in here, there are a few minor issues that will need some design consideration after this point. They can be treated as their own separate PRs.

For example the Steam wishlist element looks bad at certain small resolutions, but I don't think there's much I can do about it.

For some reason the browser doesn't capture the fixed background on the Features section but it is still there!

[tsa96]

Unable to merge without creating a merge commit, which we have disable org-wide. I don't mind merge commits as much on a small repo like this, but was going to do the postcss config stuff now anyway, so I'm going to clean up the branch and do that work on top, then get everything in in one PR. In future, please rebase instead of merge commits!

[tsa96]

Closing as superseded by #14

Really appreciate the work here. If you want to keep working on it go ahead, otherwise I can do the final styling as we're preparing to deploy this site.