Rename OpenAppStack to Stackspin

• Every occurrence of openappstack and oas in this project's codebase to stackspin
• Every occurrence of Openappstack, OAS and OpenAppStack to Stackspin
• Write a small blog article about the renaming
• Include the new icon
• Move site to stackspin.net
• Add redirect from openappstack.net to stackspin.net
• Add main branch and make it default
• Remove master branch
• Move Newsletter to newsletter.stackspin.net