CQL UI for Content picker should include "Current page"

(from Natalie's comment in CONF-34326)

In the CQL UI Space picker the user is given the option of "Current space" which creates underlying CQL like "space = currentSpace()", but for the Content picker there is no corresponding "Current content" that creates CQL like "parent = currentContent()".

Given that the currently-active content is already in the same internal Context object used to resolve the currentSpace() function, it should be easy enough to create the "currentContent()" function.

Then, in the UI, a "Current content" option could appear at the top of the "Suggested pages" list when a user triggers the Content picker.