Move ViewableData::Me() out of ViewableData and into the template layer

[GuySartorelli]

The ViewableData::Me() method just returns $this. It only exists as a way for the template layer to refer to the current item in scope. This should be moved into the template layer.

I originally did this as part of #11244 but I had to remove it (see https://github.com/silverstripe/silverstripe-framework/compare/f9e757a0a201799dbb1d2535c90e2cb309982026..29793a31415d01c4173cb560e7d97f399d270971) because it was causing some failures in the kitchen sink.

Acceptance criteria

• PHP logic that relies on calling the Me() method is refactored to not rely on that method.
• The ViewableData::Me() method is removed
• Using $Me in a template behaves identically to how it currently does
  • In control blocks, it references the current item
  • On its own, it tries to render the item to the template

PRs

• API Deprecate ViewableData::Me() #11249 (CMS 5 deprecation)
• DOC Add note to changelog about deprecating ViewableData::Me() developer-docs#516 (CMS 5 changelog)