Rationalize theme functions to improve consistency, including render elements

Let's start with #1183250: Add a theme_datetime() function to consistently theme dates and datetimes once the basic functionality is committed.

See #1183250: Add a theme_datetime() function to consistently theme dates and datetimes from comment 191 on for a stub patch and an explanation of this issue.

Basically, theme_datetime does not yet account for render arrays; it will only accept plain text or HTML strings until it is updated to incorporate the patterns of the render system.