[promos] integrate AffirmPromo module

[style] newline
[build] be more specific about symlink

link the Affirm module subdirectory, not the parent namespace directory
[docs] change title (settings menu)
[config] sort menu items
[defaults] let api url default to sandbox
[defaults] provide defaults for min and max order totals
[promo] provide helpful defaults for position/size/enabled
[loan] expand config menu by default
[config] remove merchant api key from promo config menu
[config] expand promo config menus
rename config menus
[promo] use api key from Affirm_Affirm module
[docs] don't include product-specific info in menu title
[promos] move container settings to developer settings menu
[loans] rename config menu items
[docs] update configuration instructions
[loans] bump version
[promos] bump version (initial public release)