Exchange simplification

[SchrodingersGat]

Simplification of currency exchange rates.

Ref: #1482

[SchrodingersGat]

@matmair @eeintech this PR represents somewhat of a regression in terms of functionality, but the way that currency exchange works now has been greatly simplified.

The currency "settings" view now looks like this:

Notes:

• There is no facility to manually edit exchange rate data (this will be added back in later after more of a case study)
• Currency settings are now reverted to being configured in the settings file rather than database settings (to work more cleanly with django-money)
• Simple button to "update now"

[eeintech]

Thanks for the quick fix @SchrodingersGat

Minor things but worth mentioning:

• Changing the base currency in the config.yaml does not reset the exchange rates (which could lead to erroneous conversions)
• There is a leftover print here:
  

  InvenTree/InvenTree/InvenTree/views.py

  Line 786 in 59aca27

  print("POST!")

[SchrodingersGat]

Thanks @eeintech - #1612