Instead of noting users that are already in a cohort and not changing
such users, clobber the previous cohort and display a message indicating
the previous cohort membership.

JIRA: FOR-452

Instead of noting users that are already in a cohort and not changing
such users, clobber the previous cohort and display a message indicating
the previous cohort membership.

JIRA: FOR-452

Have to return false from event handlers to avoid href="#"
links from sending the user to the top of the page.

- still needs style, more features, but basic functionality works

- lets user see and add cohorts
- needs styling
- needs to allow them to actually manage membership!