Added complexity rule for api query methods [warn]

- We want to keep behaviour in services and libraries, not in API endpoints
- This rule runs complexity _only_ on the query methods, and has it set super low - just 3
- Methods that have higher complexity are a great indicator of places where we've left behaviour in the API, however!
- It's indicative, not definitive. At least with an eslint rule we can if needs be disable it where we decide the code is OK

.eslintrc.js

@@ -16,6 +16,12 @@ module.exports = {
         'one-var': [2, 'never']
     },
     overrides: [
+        {
+            files: 'core/server/api/canary/*',
+            rules: {
+                'ghost/ghost-custom/max-api-complexity': 'warn'
+            }
+        },
         {
             files: 'core/shared/**',
             rules: {