retain_repo_version as system wide setting

[simenon]

Is your feature request related to a problem? Please describe.
Currently the default retain of repositories is null, which is unlimited. There is afaik no option to override the default system wide in the settings. Each individual created repo must explicitly set retain_repo_version if you want to deviate from null

Describe the solution you'd like
Add a system wide setting in setting.py to provide an alternate default value for retain_repo_version

Describe alternatives you've considered
Create every repo manually with the wanted default

Additional context
none