ugly gallery in feeds

[andy]

gallery shortcode should output simpler markup in feeds

[ryan]

[7298]

[gziolo]

In 59238:

Editor: Bootstrap block binding sources with inline script from server

Bootstrap block bindings sources earlier in the process through an inline script to ensure they are available when developers want to extend them in the client. Following the same pattern other APIs like registering block types are doing.

Props santosguillamot, cbravobernal, gziolo.
Fixes #6225.