Entity browser iframe appears over tour module tours

Problem/Motivation

Entity browser iframes appear above tour module tours:

z-index originally added in #2808017: Move CSS for general layout of browser widget from File Browser module to here → , no discussion around why it was set so high.
tour module uses z-indexes of ~100, there seems to be no reason for ~500 used in entity_browser

Proposed resolution

Lower z-index in entity_browser.iframe.css

Remaining tasks

Code, test, approve.

User interface changes

Tours will appear above entity_browser iframes

API changes

N/A

Data model changes

N/A