Team browser only displays active team members

[mtorres253]

So that people can accurately see who is currently working at 18F, there should be a way for only active team members to be displayed in the team browser

Acceptance criteria
Given I am an 18F employee
When I am viewing the Team Browser Homepage
Then I see only active employees (employees whose end date is not in the past)

Related story
https://github.com/18F/team-api.18f.gov/issues/244