Option to include Signed-off-by
### Problem to solve
I want to know not only the author of a change, but also who reviewed it and approved. This is not possible without having access to the original project and GitLab instance to dig up the merge request. This is difficult, and even with a merge request link, may not be possible.
### Further details
### Proposal
When creating a merge commit, or merging using the **Fast forward** merge method, include `Signed-off-by:` metadata in the commit
```
Merge branch 'sy-service-selector-refactor' into 'master'
Refactor dashboard ServiceSelector to use consistent selection API
See merge request !23488
Signed-off-by: Stan Hu <stan@gitlab.com>
Signed-off-by: Vitali Tatarintev <vtatarintev@gitlab.com>
```
### Links / references
issue