new Product UI + add MasterVariant class

• [fixed] loading drops constants in production
• [fixed] fix translations - remove  
• [added] add MasterVariant drop
• [added] add MasterVariant class and :master association to Product
• [changed] better UI - Variants in separate tab, inline variant adding, etc.

Rails 5 support

• [added] added english translations
• [changed] Rails 5 support