Enhancement/Single class classification

[c-w]

Doccano's text classification project type currently supports multi-class classification scenarios, i.e. each document can be annotated with N of M labels. However, sometimes we require single-class classification, i.e. each document can only be annotated with 1 of M labels.

This pull request adds an option to mark a project as "single class classification" which enforces that only a single label can be added to each document.