Distinguish original post from replies in HTML

Wrap post elements in .flow-post-main

There are a variety of times when the javascript wants to replace a post,
but only has information about that specific post and not all its decendants.
This patch separates the post from its replies by wrapping it.

e.g. we previously had:

<div class="flow-post">
    ...
    <div class="flow-post">...</div>
</div>

and we now have

<div class="flow-post">
    <div class="flow-post-main">
        ...
    </div>
    <div class="flow-replies">
        <div class="flow-post">...</div>
    </div>
</div>

With this new structure the javascript can replace flow-post-main and leave
all the nested replies intact.

Change-Id: I1e9d1208ae9f22e91390b0930b9e055402ca5bb4