This model and the corresponding table will be used for storing settings
specific to CI/CD, starting with the "group_runners_enabled" boolean.

The model is called ProjectCiCdSetting and not ProjectCiCdSettings. The
project exporter doesn't like plural model names as it uses "classify"
which turns those into singular (so "ProjectCiCdSettings" becomes
"ProjectCiCdSetting", producing an error if said class is undefined).

The initial work in this commit was done by Dylan Griffith, with most of
the migration work being done by Yorick Peterse.