Warn site builders about markup loss when changing the editor of a text format

Problem/Motivation

#2166399: Changing CKEditor configurations by changing text formats causes markup to be lost: warn the user when performing this advanced action added a warning for editors when changing text formats while editing content.
Changing the editor of an existing text format has the same destructive possibilities, but for all content using that text format.

Proposed resolution

Emit a similar (but more scary?) warning on the text format configuration screen when the editor is being changed.

Remaining tasks

1. Figure out the wording of this warning
2. Write the code to display it

User interface changes

A new warning will be displayed when changing the editor of an existing text format.

API changes

N/A

Data model changes

N/A

Release notes snippet

TBD