What kind of change does this PR introduce?

• Refactor

Does this PR introduce a breaking change?

• Yes

UUID (e.g. used in guid columns) must actually be valid UUIDs, meaning

• valid format and characters
• unique at least within any given table

Applied to

• contentcontainer.guid
• content.guid
• file.guid
• space.guid
• user.guid

The PR fulfills these requirements

• It's submitted to the next branch, not the master branch if no hotfix
• When resolving a specific issue, it's referenced in the PR's description (e.g. Fix #xxx[,#xxx], where "xxx" is the Github issue number)
• All tests are passing
• New/updated tests are included
• Changelog was modified

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: