Placeholder: SilverStripe 4 compatibility

[robbieaverill]

This is a placeholder issue for tracking progress towards SS4 compatibility

Requirements

• [x] Branch current master off for ongoing support of current version
• [x] Update composer constraints and branch alias: #150
• [x] Rename code structure in preparation for namespacing and PSR-4: #150
• [x] PSR-2 style compatibility, one class per file, etc: #151
• [x] Add namespace and implement throughout code: #153
• [ ] Rename templates if required: #153
• [x] Add a few unit tests
• [ ] Update CMS button styles for Bootstrap 4: #155
• [ ] Update documentation