It used to insert block Oneboxes inside paragraphs which resulted in
invalid HTML. This needed an additional parsing for removal of empty
paragraphs and the resulting HTML could still be invalid.
This commit ensure that block Oneboxes are inserted correctly, by
splitting the paragraph containing the link and putting the block
between the two. Paragraphs left with nothing but whitespaces will
be removed.
Follow up to 7f3a30d79fa63aca063e4186a21f983e23d759e8.
This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
