Introduce Artifact Expiration Defaults
Description
Presently GitLab supports setting a expiration times on artifacts, but it is optional and must be specified. This can lead to projects consuming significant amounts of disk space, if the CI user did not know or care to use this feature.
We need to introduce a way to control this behavior and set sane defaults, that can then be overridden.
Proposal
We should have an option to control this, and set a sane default value. We could add this option to the existing System Settings configuration flow, as we already have another option to control the maximum artifact size. 
We could introduce this as another option underneath, controlling the default expiration of artifacts as well.