curating community: "reject" button should ask for a confirmation and should ask for feedback to the author

[cruzki]

I am a curator in a group and when I reject a contribution I miss being able to send feedback to the author. Moreover, this way it would prevent hitting the button by mistake as this is unrecoverable action. This is related with #1676 but not the same. Please take into consideration that you can ask for some clarification (ask for feedback) but also wanted to give reasons to never accept a contribution.