Setup / clean up repo

[mkurz]

• Rename branches
• Instead of folders for versions use branches per version
• GitHub actions
• Copyright header
• Change groupId to org.playframework
• Do not forget to change the groupId everyhwere (docs!)
• sonytype settings
• Scala steward
• more...

btw. from the original maintainer:

By the way, the project was developed using a “core” that was designed to allow the use of different libraries, not just Bootstrap.