Add correct markup for Brid.gy quote tweets
As noticed with #864 (closed), the markup for a quote tweet with Brid.gy isn't quite right.
According to gRegor, all we should need to do is add the link to the
post in the e-content
(as per
https://gregorlove.com/2019/12/i-was-going-into-tosche/). However,
because the e-content
is separately generated, this isn't working.
We can put that into a separate partial so as not to duplicate code.
Closes #864 (closed).