[Sprint/NuancedNumbat](fix): Stop container overflowing horizontally

Testing

To see the original bug on production:

1. Go to group, narrow your window so that its like that of a tablet browser.
2. Scroll down the newsfeed in H&S and keep scrolling.
3. After 10-15 reloads the bug should appear (I believe at post 150).

This actually seems to be the simplest fix possible. I don't think the sidebar is the issue in this instance as initially though.

Need to check on sandbox with my group of 150 posts before merge, will link here when ready to check.

Closes #1477 (closed)