Add copy states to DesignCollection

What does this MR do?

This MR adds a new copy_state state machine to DesignCollection.

The copying of designs from one issue to another happens async (in a downstream MR !41714 (merged)), so the state of the copy is tracked through the new state.

The state is persisted in redis as the DesignCollection model is a PORO and not persisted in PostgreSQL.

This will allow the frontend (in a future MR) to display a "your designs are on their way" message until the copy has completed, and handle the error state #13426[error_moving_designs.png] by informing the user that an error happened, allowing them to upload new ones.

• MR that uses this new state: !41714 (merged)
• Issue: #13426 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done