Enabled error for "no-shadow" eslint rule

refs 635580f291

- Changed `no-shadow` eslint rule from `warn` to `error` so we don't re-introduce shadowed variables in the future. Should be removed once this change is introduced in a base plugin
- Referenced commit is a chain of commits which fixed the warning which this rule was producing previously

.eslintrc.json

@@ -10,6 +10,8 @@
         "plugin:ghost/node"
     ],
     "rules": {
+        // TODO: remove this rule once it's turned into "error" in the base plugin
+        "no-shadow": "error",
         "no-var": "error",
         "one-var": [2, "never"]
     }

test/.eslintrc.json

@@ -12,6 +12,9 @@
         "plugin:ghost/test"
     ],
     "rules": {
+        // TODO: remove this rule once it's turned into "error" in the base plugin
+        "no-shadow": "error",
+
         // these rules were were not previously enforced in our custom rules,
         // they're turned off here because they _are_ enforced in our plugin.
         // TODO: remove these custom rules and fix the problems in test files where appropriate