Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.
Some Kanboard versions return is_active and similar fields as
numeric 0/1 instead of string "0"/"1". Update UnmarshalJSON to
try string, then number, then bool parsing.