Blog: Sets 'Blog Posts' as the 1st CMS tab

As the most used and most important tab, it deserves to be the first one so users have instant access to their list of posts. :)

P.S. Sorry for the influx of PRs! If you haven't guessed, I'm putting together a SilverStripe blog package, that centres around your excellent module. It's mainly designed for use on sites for my clients, and it includes all the main modules, plus a bunch of custom tweaks to further polish the experience of each one. I've been PRing any tweaks that I feel fit best in the core.

You can see my package and list of included tweaks here: https://github.com/purplespider/silverstripe-mypswd-blog-tweaks

More than happy to create PRs for any tweaks listed that you feel would make sense in the core.

Thanks again!