Add a checkbox per issue on index (home, search results) pages and a button
that, when pressed, shows the same view but with only the previously selected
issues on display.

Hopefully this will enable us to cherry pick search results, bookmark arbitrary
issue sets and sets the foundations to allow merging result-sets.

The last chunk of the patch is pretty crude, making it better should also give
us stable "Sort on" pre-selection on displayed issue sets.