Refactor currency class

[studybuffalo]

• Refactoring the Currency class to make use of class methods to improve stability and testing.
• Reworking currency declaration to allow both a string declaration or a custom dictionary.
• Updating settings docs with details around new functionality.

[codecov[bot]]

Codecov Report

Merging #185 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #185   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         9    +1     
  Lines          722       801   +79     
  Branches        35        47   +12     
=========================================
+ Hits           722       801   +79     

Impacted Files	Coverage Δ
subscriptions/currency.py	100.00% <100.00%> (ø)
subscriptions/forms.py	100.00% <100.00%> (ø)
subscriptions/templatetags/currency_filters.py	100.00% <100.00%> (ø)

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