Make post content autosave work more generically

[helen]

The JS for autosave/AYS looks for the contents of #post #content. While we should fix the other JS issue related to targeting #content in #22600, it seems that autosave/AYS should be looking for the textarea (or whatever type of input) with the name of content instead, since having that data in the form will save to the post content. That way, if somebody does choose to use a different ID but the right input name, the autosave benefits will kick in.

Discovered while working on #22491.