Smaller Church Online section on watch page for small displays

The Church Online section on the watch page for small displays took up
quite a lot of vertical space; to increase visibility of the On Demand
section, this Church Online section was reduced in height.

The background image was also updated to better match the on-demand
visual for the latest series.

Changes to be committed:
modified:   images/watch/watch-live-small.jpg
modified:   images/watch/watch-live-small@2x.jpg
modified:   scss/pages/_watch.scss
modified:   stylesheets/app.css