ghactivity_team shortcode: fix so it can display info for teams

[jeherve]

https://github.com/jeherve/ghactivity/blob/master/shortcodes/team-activity.php

The shortcode does not currently work, it would need to be fixed. We also have the opportunity to get rid of a lot of duplicate code, that already exists in https://github.com/jeherve/ghactivity/blob/master/reports.ghactivity.php

[brbrr]

Fixed in #10