restore insert mention
for this, we need to store the last caret in the editor,
to know where to insert the user pill.
Because clicking on a member blurs the editor, and the
selection is moved away from the editor.
For this reason, we keep as cache of the last caretOffset object,
invalidated by a selection with different values.
The selection needs to be cloned because apparently the browser
mutates the object instead of returning a new one.
