As OWNER of an organization, I want to add an exisiting user as a member.

[uliSchuster]

I authenticate and launch the Clair Dashboard UI. I select the desired organization in the organization drop-down. If and only iff I am an OWNER of the selected organization, a menu item "add organization member" appears. If I select this menu item, a dialog appears that prompts me for the desired user name or email. This is not a drop-down or type-ahead box. I need to know the identifier and enter it in full. When I click "Add", the dashboard checks if the user is known, and if so, submits a request to the membership API endpoint.

There is no opt-in, the user is added directly.
It might be sensible to inform the new member that he or she was added to an organization. This is subject to [another ticket](ClairBerlin/managair#97.

If there is an organization overview, the member of the organization should be displayed somewhere.

Complexity: 5
Value: 20
WSJF: 4