Refactor TopPage to use the standard dir structure

[emteknetnz]

TopPage has a 'feature based' directory structure, which doesn't match anything else in Silverstripe core modules

While the argument can certainly be made that feature based dir structures are a better way to organise the code, it this context it's simply non-standard.

Acceptance criteria

• Move the files in the TopPage dir to other standard locations
• Update namespaces in moved files
• Update referenced namespaces in config files
• Appropriate classes are deprecated in CMS4, though wrapped in Deprecation::withNoReplacement() since the files aren't availabe until CMS5
• Changelog is updated