✨ Added custom count queries for "max" limits

refs https://github.com/TryGhost/Team/issues/597

- When the library is used on a client without a DB connection (e.g. frontend client running in a browser) the library needs to expose a way to override count queries.
- The way these can be used is giving a count based on a HTTP request or some other data provider
- Example use with max limit like "staff" would be loading the limit servcie if following way:
```
            const limitService = new LimitService();

            let limits = {
                staff: {
                    max: 2,
                    currentCountQuery: () => 5
                }
            };

            limitService.loadLimits({limits, errors});

            await limitService.checkIsOverLimit('staff')
```

ghost/limit-service/lib/limit-service.js

@@ -28,7 +28,7 @@ class LimitService {
 
             if (config[name]) {
                 /** @type LimitConfig */
-                let limitConfig = _.merge({}, limits[name], config[name]);
+                let limitConfig = Object.assign({}, config[name], limits[name]);
 
                 if (_.has(limitConfig, 'max')) {
                     this.limits[name] = new MaxLimit({name: name, config: limitConfig, helpLink, db, errors});
@@ -97,4 +97,5 @@ module.exports = LimitService;
  * @prop {Number} [max] - max limit
  * @prop {Boolean} [disabled] - flag disabling/enabling limit
  * @prop {String} error - custom error to be displayed when the limit is reached
+ * @prop {Function} [currentCountQuery] - function returning count for the "max" type of limit
  */

ghost/limit-service/test/limit-service.test.js

@@ -109,4 +109,29 @@ describe('Limit Service', function () {
             limitService.isLimited('customThemes').should.be.true();
         });
     });
+
+    describe('Custom limit count query configuration', function () {
+        it('can use a custom implementation of max limit query', async function () {
+            const limitService = new LimitService();
+
+            let limits = {
+                staff: {
+                    max: 2,
+                    currentCountQuery: () => 5
+                },
+                members: {
+                    max: 100,
+                    currentCountQuery: () => 100
+                }
+            };
+
+            limitService.loadLimits({limits, errors});
+
+            (await limitService.checkIsOverLimit('staff')).should.be.true();
+            (await limitService.checkWouldGoOverLimit('staff')).should.be.true();
+
+            (await limitService.checkIsOverLimit('members')).should.be.false();
+            (await limitService.checkWouldGoOverLimit('members')).should.be.true();
+        });
+    });
 });