Correctly align resolved discussion text

The header of the `NoteableDiscussion` component is really a media
object[1], but it didn't have the right DOM or styling to correctly
achieve it.

This change wraps all the children of the media object (except the
floated image) in a dedicated `.timeline-content` element, which is
styled to establish its own block formatting context[2]. This ensures
*all* of its children sit left-aligned to the right edge of the float.

In short, the DOM used to look like this:

    .discussion-header
        .timeline-icon
        note-header
        note-edited-text

and now it looks like this:

    .discussion-header
        .timeline-icon
        .timeline-content (with own formatting context)
            note-header
            note-edited-text

This is also now more consistent with the DOM of `NoteableNote`, in that
`.timeline-icon` and `.timeline-content` seem to want to be siblings.

[1]: http://www.stubbornella.org/content/2010/06/25/the-media-object-saves-hundreds-of-lines-of-code/
[2]: https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Block_formatting_context

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