[Team] Switch between multiple teams via dropdown

[rnavyaprabha]

Description:
As a user who is a member of multiple teams, I want to switch between teams using a dropdown so that I can view each team's feed and activity separately.
Acceptance Criteria:

A team selector dropdown is accessible from the Teams page (and ideally the top nav)
All teams the user has joined or created are listed in the dropdown
Selecting a team updates the page to show that team's data, feed, and collaborators
The currently active team is visually indicated
Works for both admins and regular members