Allow multiple selections from the media library in CKEditor

Problem/Motivation

This is spun off from #2994699-76: Create a CKEditor plugin to select and embed a media item from the Media Library and subsequent feedback, where @Aaron McHale asked:

is there a reason the user is limited to inserting only 1 item? The Media Library does technically support inserting as many items as it's configured to, I could totally see a scenario where someone is writing an article and wants to insert a several images, but currently they'd have to open the Media Library, upload or select an item, close the Media Library, and then repeat several more times. A more efficient workflow would be if there wasn't a limit of 1 item and so the user could both upload and select, then insert all the media they needed in essentially one operation.

Interesting idea. This issue is where we should look into the possibility of implementing this, and if we do decide to, let's work on that here.

Proposed resolution

TBD. But first, let's port the work from #2994699-77: Create a CKEditor plugin to select and embed a media item from the Media Library over here, so that we don't throw out the proof-of-concept code that @oknate wrote.

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD