Convert template to a G8 one

[aadrian]

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.