Update slug to be unique together with certifying organisation

[sumandari]

This is to fix #1357

When user create new course type, it will create slug with 50chars max from course name.

This PR enable the different certifyng organisation to create the same course name.

[sumandari]

Notes:

• When user create a new course type, it will create a slug with 50chars max from CourseType name. If the course name is longer than 50 chars, it will lead to a slug duplication.
• We have applied a mechanism to check and add counter aftername when it exists and create a slug from it. But the unique field is slug instead of name.

In the issue #1375, there is an existing courseType with same slug but it has different name. I guess the existing courseType was edited with a new name, and that's why our code cannot prevent this duplication. So I think we can change the way we add counter in the slug duplication rather than change the unique value. Would you have suggestion @dimasciput?

[sumandari]

We are not implementing this. I will close this PR