[#882] For the Bold: tweak poster positioning on small screens

* add a right margin to the poster (but only on small screens) so that
  it won't overlap the userpic

* keep the negative margin top so there's something beside the userpic

* some refactoring so we can calculate the required right margin

Fixes #882.
Merge pull request #897 from afuna/for-the-bold-icon-overlap-bug-882

[#882] For the Bold: tweak poster positioning on small screens