* Adds a new webpack entry point for references previews
* Reference related code in src/index.js is moved to new
file resources/ext.popups.referencePreviews/index.js
The changes:
* References previews now in its own module ext.popups.referencePreviews
* Loaded via getCustomPopupTypes
* OWNERS.md files make clear which team owns which part of the code.
Bug: T326692
Change-Id: Iea8a5b9221c0b1fd41e40bff2cbe01e42124d53f
