Show a warning when a content area contains pasted MathJax output

[christianp]

MathJax uses id attributes on its output to identify expressions that it's typesetting, with a counter that increments from 0 after the page loads.
So if you paste output from a previous run into a content area, MathJax will eventually get to the same id attribute as it processes TeX in the page, and throw an error when it finds the pasted element instead of the one it's just created.

The editor should detect elements with MathJax attributes, starting with MathJax-, and show a warning.

[christianp]

This is really the same thing as #648.