UserDefaults can only be configured by admins

[ingoj]

The plugin only alows admins to configure the plugin (by checking if globel role 2 is held by the current user.
It should be possible to configure the plugin for non-globel admins (other plugins often do not check anything and rely on visibility of plugin adminstration to the user).

PR #25 addresses this and checks, if the current user is allowed to change role assignments in user adminsitration. Since most functions of UserDefaults (de-)assign roles to users, this could be an adequate behviour of the plugin.

Other solutions are defintiively possible and appreciated.

REST API access is not touched by the PR and still require global admin role (could be changed easiy)