Allow adding and switching teams

[svenefftinge]

Users shall be able to create teams. And switch the app context between their account and teams on the top left of the dashboard. A team consists of a name, that needs to be unique within the db (including user names) and consists of URL fragment characters. We also need a blacklist of names that we don't allow because they conflict with pages in the dashboard (e.g. settings, workspaces, admin, etc.).

A team is reachable by its URL which is https://gitpod.io/<teamname>

User Experience goal

The user should be able to create new teams and switch between teams through the dropdown element.

Proposal

In this first MVC (minimum viable change), we can do the following:

1. Update the top navigtation with tabs so that Workspaces and Settings are palced below the account dropdown.
2. Move the Admin item to the right menu.
3. Introduce a dropdown to switch between teams as well as create new teams.
4. Introduce a full page form to create a team.

Designs

Default	Dropdown	New Team	New Team (Error)

See also design specs.