Improve the Currency storage and UX

Problem/Motivation

Currently, users have to manually add currencies and set currency ISO codes as currency ID. This is far from ideal. Ideally, Currency should contain more fields (listed in the "Data model changes") and be more easily added.

Proposed resolution

Enhance the currency configuration process by providing a list of all currencies with an 'enabled' state instead of manually typing currency codes in the ID field.

Remaining tasks

No work done yet.

User interface changes

On the currency add page, a select element with all available currencies should be present.

API changes

API should provide not only "convert" method, but other methods for retrieving currency metadata (new fields).

Data model changes

These fields may be added to Currency:

• Name
• Symbol (abbreviation)
• Fractional unit
• Number to basic
• State or territory