Add a media / gallery page

[pavlos256]

This is different from #51.

It should have a few gameplay videos (not trailers imo) and a few choice screenshots. Also links to youtube channels / playlists.

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[Mailaender]

We currently don't have too many recent and good looking screenshots. I suggest we collect more material at http://www.moddb.com/games/openra/images. Everyone should be able to upload there.