Revoke my own access to an instance

[jml]

Follow-up from #581

Given that:

• anyone can invite me to any instance they have access to
• the title of the instance can be something abusive/confusing/threatening
• the UI always shows all of the instances that I have access to, with no way of organizing or filtering
• having access to an instance means access to all of the computers in the instance, which makes me a risk to others

I should be able to revoke my own access to an instance.

If we do this, we need to handle the case where the last user revokes their own access to an instance. Options include:

• delete it
• don't let the last user revoke their own access

[jml]

The API component for this is already done (#661 DELETE /api/users/org/{orgName}/users/{myUserEmail}). Only UI work remains.