Add ability to import transactions

[jaydrogers]

Import CSV

• [x] User clicks "Import"
• [ ] Drag and drop CSV
• [x] Create GUI to select which fields to import and validate mapping
• [ ] Show loading state
• [x] Show success notification

International support

• [ ] Allow support of international currencies on a per account basis

[etienne-monier]

Isn't it easier to import OFX bank files?