Error with .load() calls in Omeka 3 and jquery 3

[apjmason]

Exhibit page edit screens are throwing "e.indexOf is not a function" errors.

Omeka 3 upgraded to jquery 3 which deprecated the .load() method

Per that page, we need to replace the calls to .load() in OmekaFootnotes.js with .on("load",callback)..

[apjmason]

@tutingjun this should be a relatively quick fix. Can you and @AlejandroG31 verify the issue first and issue a PR with a fix when you have a chance?