Mattermost Plugin: default sidebar to Active Meetings if channel is linked to a team

[jordanh]

After a team has been linked to at least one team, let's default the plugin sidebar navigation to Active Meetings. That'll be better UX, as linking a team is likely to only happen once.

Acceptance Criteria (optional)

Users can:

• When a user is opening the side bar on a channel without teams, default to Link Teams
• When a user opens the side bad on a channel with Linked Teams, default to Active Meetings

Estimated effort: 1 hour