Reduced execution flow dependencies and made all search parsers share
the basic code rather than implementing everything all over again in
each parser through awkward protected functions.
Rather than having "creation time", "upload time", "registration time"
etc. I think it is better to have a single "creation time"
entity-agnostic property (like the one Tags had thus far).
Before this change, it was using edit time, which resulted in bumping
old posts every time a comment was edited. Users reported this behavior
as awkward and unintuitive, so I've changed it.
