ghactivity_team shortcode: Display info for teams

[brbrr]

Fixes #4

Besides fixing shortcode issue I did some refactoring and fixed few linting issues

To test:

• Make sure you have enough GH events tracked, and there at least 1 team created
• Create a new page with [ghactivity_team team=$team_name] shortcode
• Make sure it renders valid numbers

[brbrr]

Thanks!

Merging this