Fix the positioning of the "Toggle-discussion" button

The following discussion from gitlab-ce!28440 should be addressed:

• @dpalubin started a discussion: (+3 comments)

  @pslaughter I like the idea of using flex box instead of using margins/padding to center content. However, with the way the hierarchy of elements are set up, applying the flex box here will also cause the Toggle-discussion button to shift when resolving discussions.

  

  

  What is your opinion on this? I would prefer the button to remain in the same position in both cases. I liked the appearance of having the button on the top right regardless of the position of the text to the left, as in this image:

  

  This was the way it was showing up for me through the gdk in this merge request. To achieve this same effect using a flex box, the toggle discussion button would have to be taken out of note-header-info and moved to a higher level element