Adds multiple languages and verifies emails

Features:

• Sends Email to verify the account (expires after 7 days => account not usable)
• Adds language support for multiple languages with i18n: englisch, german and portuguese
• Improves datepicker component
• Adds revenue to dashboard (for each bike model)
• Improves go methods and sql queries
• Generates random test data (f.ex. orders and orderitems) for dashboard