Fast-forward merge button
For #337, #338
- [x] fast-forward merge option in project settings
- [x] fast-forward merge button
- [x] message if ff is not possible
- [x] rebase button if ff is not possible and rebase is enabled
- [x] rebase button depends on ff-only setting
- [x] remove `rebase enabled by default` checkbox from project settings
- [x] implement rebase feature
- [x] write tests
- [x] update CHANGELOG and documentation
- [x] mention in 8.0 blog post
See merge request !485