New field is added to settings to receive email of the admin of the
instance. source_code is a link to the repository of the source code
of a GitPad instance, set_source_code is defined to build a HTTP
link to the exact commit from which the instance binary was built.
Current Settings::set_source_code assumes the provided HTTP link to
point to a forge belonging to the GitHub-family(GitHub, Gitea and
GitLab), provisions must be made to accommodate other forges as