Fix #204 - Duplicated postboard-news element

New posts are announced with the .postboard-news button. But home.html also contains a forgotten div with the same css class, so all the same rules apply to it - the new_posts string is filled into it too. This isn't noticed normally, as it is set to display:none, but it can probably be accidentally selected by jQuery and called to “fade in”, causing the duplication glitch that @dryabov noticed.

Fixes issue #204.