RBAC: Explain why org role selection is disabled for externally synced users (#72274)

* Move builtin role selector to separate component

* Show message if basic roles picker disabled

* Show explanation in OSS

public/app/features/admin/UserOrgs.tsx

@@ -209,6 +209,8 @@ class UnThemedOrgRow extends PureComponent<OrgRowProps> {
                   roleOptions={this.state.roleOptions}
                   onBasicRoleChange={this.onBasicRoleChange}
                   basicRoleDisabled={rolePickerDisabled}
+                  basicRoleDisabledMessage="This user's role is not editable because it is synchronized from your auth provider.
+                    Refer to the Grafana authentication docs for details."
                 />
               </div>
               {isExternalUser && <ExternalUserTooltip lockMessage={lockMessage} />}