Allow users to make use of custom fields for both Gifts and Constituents in Letter Box.

V1 - RELEASED!

• Make fields available manually in editor
• {{custom.category.value}}
• {{custom.category.date}}
• This enables the user to format the fields with functions such as date/currency formatting + future functions

V2
Add dropdown in editor with available fields values

Only allow custom fields with one category
Load all custom fields per Constituent/Gift