Set wpActiveEditor on DOM ready

[kovshenin]

We currently set wpActiveEditor on click inside a .wp-editor-wrap, but we should also have a default active editor set on DOM load, to handle media insert from elements outside of the editor wrap, or ones triggered by non-click events, such as drag and drop in #19845.

[kovshenin]

Patch in 27210.diff​

[azaozz]

In 27270:

Set wpActiveEditor to the id of the first instance on init, props kovshenin, fixes #27210.