Add title as placeholder, prefill content with user mention

Before this patch, the textarea's placeholder is "Reply to "$1""
This patch will update that to:
* Reply to "topic title"

This will also prefill the textarea with the username you're replying to:
* 127.0.0.1:
* [[User:Mlitn|Mlitn]]:

Change-Id: Ic7c3dd7af2121dc0a913865c44772c810d14cf54
Merge "Add title as placeholder, prefill content with user mention" into frontend-rewrite