Make contrib group the codeowners for the whole repository

[juliogonzalez]

As title mentions.

With this, the default for PR reviews will be:

• GitHub will automatically assign the group contrib to any PR for this repository
• People in that group will get notifications (only @mbrookhuis for now)
• People in that group must then review, approve if everything is OK, then merge.

If needed, other Uyuni developers can be added as reviewers, and they will still have rights to merge.