feedlink/blogpost templates: use class instead of id

feedlink/blogpost templates: use class instead of id

There can be more than one feedlink group in a page, as well as (more
rarely) multiple blog forms, and using the same id for all of them
causes HTML validation errors. Replace the id with a class by the same
name and adjust in-repository CSS.
(cherry picked from commit 0c3b91e1f06fb357711cfa71d514f139cd8e04e3)