Group size limit
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### Proposal
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The repository size limit currently applies per project, while it can be set on the group level, it will still apply to each project separately.
It will be great to have the ability to set a global size limit on a group to limit its size and all its children (sub-groups, projects).
Example:
If I set a limit of 2Go on a group, the sum of all projects repository size cannot be greater than 2Go.
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