Fix editing of posts

https://gerrit.wikimedia.org/r/#/c/91306/ has introduced a small regression
because data-post-id is no longer duplicated on both .flow-post-container and
.flow-post elements.

This JS was still looking at the one where the data attribute was now removed.

Change-Id: I7593f8515bfcec7ddf74e22942efeb3ebdc2928b
Merge "Fix editing of posts"