feat(auth): migrate Application model from MongoDB to MySQL

- Add Sequelize migration creating the 'application' table
- Add Application Sequelize model with slugify, toJSONFor, toAuthJSON
- Rewrite passport-headerapikey strategy to use DB.Application
- Rewrite applications.routes.js to use Sequelize (findAll, count,
  build/save, destroy) instead of Mongoose
- Fix pre-existing bug: passport was querying { encryptedKey } but the
  stored field is named 'apikey'
- Add Application to MySQL models index and relationships
- Remove Application from Mongoose models index and delete the schema

src/database/models/mongo/index.js

@@ -1,4 +1,3 @@
-exports.Application = require('./Application');
 exports.Aeronef = require('./Aeronef');
 exports.Canopy = require('./Canopy');
 exports.Dropzone = require('./Dropzone');