Allow deleting an organization through the UI

[fflorent]

Describe the problem to be solved

The problem is described in this issue: #1182

Describe the solution you would like

Following up the issue mentioned above, it would be even neater to propose the possibility to remove organization (or team sites) to end users, through the UI.

[almereyda]

Possible duplicate of #977.

In #455 (comment) I have just suggested to offer an admin console, where such global instance-level settings could be displayed.

[paulfitz]

It is unfortunate, we actually implemented this in our SaaS, but the only home for site-level configuration we have so far is our "billing" pages so it got put there. The billing pages aren't in grist-core because they are entangled with our billing system. If anyone were willing to design a better home for site level configuration, we'd be happy to move this functionality to it.