Reconfigured codecov config

refs https://github.com/TryGhost/Toolbox/issues/583

- removed default config so we just have separate numbers for the flags
- added threshold to flags config so we can protect against small
  changes in variance

.github/codecov.yml

@@ -4,16 +4,14 @@ coverage:
   status:
     patch: false
     project:
-      default:
-        flags:
-        - unit-tests
-        - e2e-tests
       e2e-tests:
         flags:
         - e2e-tests
+        threshold: 0.2%
       unit-tests:
         flags:
         - unit-tests
+        threshold: 0.2%
 flags:
   e2e-tests:
     carryforward: true