fix(db): register Application model in mysql.js DB initializer

Application was added to models/mysql/index.js but not to mysql.js,
which is the actual file that instantiates all Sequelize models.
DB.Application was undefined, causing the relationships to fail at startup.

src/database/mysql.js

@@ -1,5 +1,6 @@
 var DataTypes = require("sequelize").DataTypes;
 const sequelize = require('./config/sequelize');
+const ApplicationModel = require('./models/mysql/Application');
 const ArticleModel = require('./models/mysql/Article');
 //const ArticleTageModel = require('./models/mysql/ArticleTag');
 const BrandModel = require('./models/mysql/Brand');
@@ -20,6 +21,7 @@ const SkydiverProfileModel = require('./models/mysql/SkydiverProfile');
 
 const DB = {
   sequelize,
+  Application: ApplicationModel(),
   User: UserModel(sequelize, DataTypes),
   Article: ArticleModel(sequelize, DataTypes),
   //ArticleTag: ArticleTageModel(sequelize, DataTypes),