[SiteSettings] Allow site admins to change user roles on a project

[imnasnainaec]

Currently, UserManagement in SiteSettings only allows site admins to delete other users.

It would be nice to be able to

• add a user to a project; ([SiteSettings] Enable add user to project #2372)
• remove a user from a project or change their role. ([SiteSettings] Let site admins manage users on all projects #2370)

This feature should be presented through the project management component.

(May be able to re-use the project settings component for users)

[imnasnainaec]

Related: #544