Add versioning to initiatives

**Is your feature request related to a problem? Please describe.** Currently an admin can apply all the changes it wants to an initiative. There is no way to track these changes. It's not transparent. **Describe the solution you'd like** We should have a versioning system for petitions like we have for proposals and results. Changes that should be tracked : - Text changes to title, description and hashtags - State changes - Changes in authorship (especially track co-authors) **Describe alternatives you've considered** None **Additional context** None **Does this issue could impact on users private data?** None