Decreased coupling of frontend routing to server api

refs https://github.com/TryGhost/Team/issues/527
refs https://github.com/TryGhost/Ghost/issues/10790

- Frontent has to have as few as possible coupling points with the Ghost Server API. By design that point has been a "proxy.api" property that will become more and more constraint in the future based to limit the surface of frontend interaction with servers's API
- Removing `.../server/api` requires in favor of using a proxy decreases direct coupling

core/frontend/services/routing/controllers/preview.js

@@ -14,7 +14,7 @@ const helpers = require('../helpers');
 module.exports = function previewController(req, res, next) {
     debug('previewController');
 
-    const api = require('../../../../server/api')[res.locals.apiVersion];
+    const api = require('../../proxy').api[res.locals.apiVersion];
 
     const params = {
         uuid: req.params.uuid,

core/frontend/services/routing/controllers/static.js

@@ -4,7 +4,7 @@ const debug = require('@tryghost/debug')('services:routing:controllers:static');
 const helpers = require('../helpers');
 
 function processQuery(query, locals) {
-    const api = require('../../../../server/api')[locals.apiVersion];
+    const api = require('../../proxy').api[locals.apiVersion];
     query = _.cloneDeep(query);
 
     // CASE: If you define a single data key for a static route (e.g. data: page.team), this static route will represent

core/frontend/services/routing/helpers/entry-lookup.js

@@ -14,7 +14,7 @@ const routeMatch = require('path-match')();
 function entryLookup(postUrl, routerOptions, locals) {
     debug(postUrl);
 
-    const api = require('../../../../server/api')[locals.apiVersion];
+    const api = require('../../proxy').api[locals.apiVersion];
     const targetPath = url.parse(postUrl).path;
     const permalinks = routerOptions.permalinks;
     let isEditURL = false;

core/frontend/services/routing/helpers/fetch-data.js

@@ -45,7 +45,7 @@ defaultPostQuery.options = defaultQueryOptions.options;
  * @returns {Promise}
  */
 function processQuery(query, slugParam, locals) {
-    const api = require('../../../../server/api')[locals.apiVersion];
+    const api = require('../../proxy').api[locals.apiVersion];
 
     query = _.cloneDeep(query);