Email Mapping Element

Follow-up to #2854021: Send email based on element options selection
Follow-up to #2847645: Field Mapping for Handlers

Problem

• A user may want to send a different message to different emails addresses based on different conditions.
• A remote post handler wants to map a webform element's to a remote post's API fields.
• A webform's elements need to mapped to a entity's fields.

Solution

Create reusable mapping element that can used for emailing routing or mapping Webform elements to remote post or entity fields.

Tasks

• Create form element
• Create test form
• Write tests.

Form Element

• #source (aka #options)
• #destination (If #options displays select menu, if NULL displays textfield )
• #destination__size: textfield.
• #select2 (Applies to #source and #destination