Refactor RunsController#download_port to use a before_filter.

Should usually do lookups in before_filters. Also tests are added to check these
methods continue to work for downloading both input and output ports.
Refactor RunsController#input to use a before_filter.

Can reuse the one that #download_port uses. Tests also added.
Refactor RunsController#output to use a before_filter.

Can reuse the one that #download_port uses. Tests also added.