Updates after gitlab.com -> gitlab.postmarketos.org

This MR does not update each and every reference from gitlab.com to gitlab.postmarketos.org, IMHO it is not worth doing that for previous blog posts. I've only adjusted what is relevant for new blog posts (the templates) and stuff outside of blog posts.

I've added a link to status.postmarketos.org to the explore page while at it, as we made a lot of use of that during the gitlab migration and it wasn't linked yet. Also modernized more things in the "source code" page.