Allow users to change their default currency across the app using a dropdown

[lcampanis]

We will introduce a currency change dropdown, similar to the one for language - practically a copy, which is now ready.
The dropdown will make use of the existing functionality, which is solely based on the user's Locale.

The value set from the dropdown will be saved in LocalStorage, as well as in users[n].profile.currency. Just like language is.
This will have the highest precedence and will be checked before the current locale functionality.

With that in place, it should be easy to put that dropdown in the header so that each user can choose their favourite currency.

Any thoughts, let us know.