Note on locales / currencies

[jantman]

In documentation, add a note on locale/currency support. Should have a clear note near the top of the README, pointing to more detailed explanation in the built docs.

• ofxgetter
• Javascript formatting
• Python formatting
• Assumptions in DB models around currency values / min/max

[jantman]

See also #125 when implementing this. We might as well make some real fixes instead of just adding notes about it. Right now we use Python's built-in locale module for currency formatting internally to the app and in the Python-rendered templates/views. For Javascript, we use a custom function that only knows how to handle USD in en_US.

So, the implementation of this should probably include:

• Switching the Python code to use Babel as mentioned in Try using alpine-based docker image (musl locale issues) #125
• Making both the locale and the currency settings options, with the locale defaulting to the system locale if it can be determined, and if not to en_US (except if it's the "C" locale like in the Alpine images), and the currency defaulting to USD.
• Finding some method of doing appropriate currency formatting in Javascript for the UI-side things.
• After all that is done, add notes in the docs about any remaining limitations, such as database field size and decimals and anything in ofxgetter.

For the javascript side, there are a number of possibilities:

• ECMAscript's builtin Number.toLocaleString() should handle this, but apparently lacks support in most mobile browsers for user-specified locales, currencies, etc.
• This SO answer pointing to Javascript's Intl.numberformat. It seems to be relatively well-supported in modern browsers including Chrome for Android, Firefox for Android, and iOS Safari, and there's a shim for older browsers if needed.
• format.js
• https://www.npmjs.com/package/locale-currency
• http://numeraljs.com/
• https://github.com/globalizejs/globalize currency formatting

I think going with the built-in Intl is the way to go. It's as simple as:

var formatted = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(123456.78);

[jantman]

Ok, what's actually needed for implementation:

• Make both the locale and the currency settings options, with the locale defaulting to the system locale if it can be determined, or else an env var, or else to en_US (except if it's the "C" locale like in the Alpine images), and the currency defaulting to USD.
• Switch the Python code from using the locale module to using Babel's format_currency
• Replace our custom JS formatting function with Intl.NumberFormat, using the locale and currency from the settings file.
• After all that is done, add docs:
  • Note near top of README for anyone outside en_US / USD to see link to more detailed explanation in the docs
  • Document how to change locale and currency (links to settings) and how they're defaulted
  • Document assumptions in the DB models around numeric length and number of decimals, and willingness to fix this
  • Document that the currency defined in OFX files is ignored by ofxgetter/ofxbackfiller; we assume that there's only one currency in use, and it's the one defined in the settings file
  • Document that the functionality to sync Projects/BOMs with Amazon Wishlists only works for US wishlists, and items in USD.