WIP Increment total_people_joined count for team

WORK IN PROGRESS. DO NOT MERGE.

This increments a count on the Team model to track
how many people have signed up via a team page.

After successfully joining, user is shown a success page
explaining what they just signed up for.

TODO Add appropriate explanation for join form.
TODO Add appropriate content for join_team_success.html page.
TODO Fix ugly styling of join_team_success.html and
     thank_team_success.html pages.

Finishes https://github.com/MayOneUS/teams/pull/32 and #28.