Those three custom tags are added to the DEMO (DEMO-147) as examples until they can be added to the kernel.

For that to be possible, full support for the standard design must be implemented (EZP-28870). With it, the tags can be added to the richtext package using the @ezdesign syntax, and be overridden by integrators.

As the tags have been reviewed before getting integrated in the demo, they should not require extra changes besides the theme.

Scenarios

As a user, I want to embed twitter, facebook and youtube content into richtext fields
As an integrator, I want to customize the template used to render twitter, facebook and youtube embeds