fix blank profile pic dimensions, add sensible limits

change blank image to square (rather than 1x16) so that it's not
ridiculously tall when scaled up. also add min and max height for
avatar images at display time so that we don't break the layout

test plan
1. go to the new profile page on an account with avatars disabled
2. the blank avatar image should be a square

Change-Id: I68729fbe2097c893aa6623b867ee22591296ed36
Reviewed-on: https://gerrit.instructure.com/12696
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>

app/helpers/application_helper.rb

@@ -153,7 +153,7 @@ module ApplicationHelper
     if session["reported_#{user_id}"]
       image_tag "messages/avatar-50.png"
     else
-      image_tag(avatar_image_url(User.avatar_key(user_id || 0), :bust => Time.now.to_i), :style => "width: #{width}px;", :alt => '')
+      image_tag(avatar_image_url(User.avatar_key(user_id || 0), :bust => Time.now.to_i), :style => "width: #{width}px; min-height: #{(width/1.6).to_i}px; max-height: #{(width*1.6).to_i}px", :alt => '')
     end
   end