"external_sources" feature flag needs to be improved

[obulat]

Problem

"external_sources" flag name is not very clear, and the External sources popover does not use additional types when this flag is enabled.

Description

I propose renaming the flag to "additional_content_types" as it enables the display of additional types in the content switcher. When this flag is enabled, you should be able to select the external sources for the page's type (even video and model_3d). Now, however, those external sources are never displayed.

Additional context

I discovered the problem that you cannot view the external sources for additional content types when trying to remove the build-url library that's been deprecated. It is used to create the external source URLs with query parameters.