Move some fixmystreet.com-only stuff into cobrand.
[fixmystreet.com] Remove login content experiment.

This gave no conclusive results, which was interesting.
[fixmystreet.com] Experiment, no pins by default.

This changes the default flow to show no pins and a message about
reporting, with the second tab displaying existing problems (and showing
map pins). Those existing problems are the ones that would currently be
on "problems on the map" followed by those currently on "problems
nearby".