Disable UD for non-members at genesis

• Add end2end test about UD eligibility at genesis for members or non-members
• Fix non-eligibility for non-members
• Remove implicit IdtyData::new
• Make FirstEligibleUd::default explicit in doc

fixes #273 (closed)

end2end-tests/cucumber-features/universal_dividend.feature

+@genesis.wot
+
+Feature: Universal Dividend
+
+    Scenario: Eligibility at genesis
+        When 2 blocks later
+
+        # Members
+        Then alice should be eligible to UD
+        Then bob should be eligible to UD
+        Then charlie should be eligible to UD
+
+        # Not members
+        Then eve should not be eligible to UD
+        Then ferdie should not be eligible to UD