Front-end interface for the new start_date field.
New RedmineREST endpoint. Want to see the structure of an issue.
Give all Redmine tickets a "due date."

It's a bit confusing beause Redmine itself has a concept of a "start date",
but this does NOT correspond to the new hire's start date. Instead, it's
semantically better to use Redmine's "due date." Weird.
Sanitation and proper error detection for new hire start date.
Merge branch 'start_date'