Affix discussion sidebar

[tobyzerner]

From http://discuss.flarum.org/d/396/general-ux/1:

I know that adjusting the forum for all possible widths is laborious enough, but I sometimes use really small windows, height-wise too. There seems to be an arbitrary gap between the discussion info box and the top of the page. Perhaps it could be moved to top on certain heights?https://i.imgur.com/HQ5KQW2.png

What needs to be done:

• Apply the affixSidebar util as config for the DiscussionPage sidebar
• Test to make sure it doesn't muck up any of the scrubber scrollbar mechanics
• Potentially adjust the max height of the scrubber scrollbar so on tall screens it doesn't look awkward

[tobyzerner]

From #1068: I am planning some design changes for the next release which will make this unnecessary.