Deprecate cets_EmbedRSS plugin
The cets_EmbedRSS plugin is network activated on the Commons, and appears to have been network activated since the Commons began in 2009.
The plugin has not been updated (aside from in-house updates) since it was introduced into our codebase in 2010. Most recently, see #11501.
The plugin appears to do two things:
1. It adds a `show_RSS` shortcode, which fetches a feed and displays recent items
2. It adds a TinyMCE button that makes it easy to create the shortcode
Given the plugin's extreme age and unmaintained status, I'd suggest that it should not be network activated. However, it shouldn't be removed for sites that are currently using the show_RSS shortcode. So we'd need to write a script that identifies the sites using shortcode, and activates the plugin on those sites.
If there are no objections, I'll move forward with this.