Unify stream and viewer styles

- Vertically align the thread expander using ems and taking into
  account the size of the header it's aligned with.
- Fix excerpt bottom margin to match viewer
- Tuck the thread border under the content of the parent with a little
  extra padding
- Align the padding of the content class with card margins
- Rather than using a space character in the font icon styles, use the
  offset margin by choosing a sensible, non-zero default

Close #1290