Fix bug: Old drafts pop up again. And add draft indicators

Here @chrscheuer mentions that:

I still sometimes encounter that I have posted something and then when I try to answer to another post/comment then I'll get the draft up for something that was already posted.

To better understand when this happens, I think it'd be helpful with a "you have a draft" symbol below a post, for which one has started replying (so that a draft got saved). That'd be nice to have in any case I think. So I can look into adding that, as step one.