Convert template to a G8 one

Please convert this project to a G8 one like https://github.com/scalatra/scalatra-sbt.g8

so that:
 - the users can run it with e.g. ```sbt new gitbucket/gitbucket-plugin.g8```
 - has default values that users can change during the init step (right now it's not interactive at all)

Thank you.