Start instance of base class, load configuration and establish database connection. Please don't call the constructor direcly, use the singleton pattern instead.
Tests given abbreviation long_form for uniqueness. Takes the abbreviation long_form as first argument and an optional abbreviation id as second argument. If the abbreviation id is given, this abbreviation won't be considered when checking for uniqueness (useful for updates).
Returns boolean true if abbreviation long_form is unique.
