POS Enhancements

[kaloudis]

This PR adds the following changes to the POS:

• Currency not set error message does not block rendering of the rest of the fields and only displays if Square is selected. This allows users to use the Standalone mode without setting a fiat currency
• Fixes a bug where the keypad and the printer could not be enabled at the same time
• Adds placeholders for when no categories or products are defined when in Standalone mode
• Adds loading indicator to Products and Categories view in Standalone mode
• Hides the fiat rate on the Standalone POS view if fiat rates aren't enabled
• Show charge and clear buttons even if no products are defined - this will allow people to clear orders if they're using the keypad w/o defined products

[shubhamkmr04]

tACK