Check if project-slug exists when creating a project

[oliverroick]

When creating a project with a name that already exists an IntegrityError is thrown, because the slug must be unique:

IntegrityError at /projects/new/
duplicate key value violates unique constraint "organization_project_project_slug_key"
DETAIL:  Key (project_slug)=(xls-project-3) already exists.

When creating a project, it should be checked if a project with the same slug already exists and then change the slug accordingly.

[seav]

This bug is still occurring on the latest master. I have created a functional test for this specific bug: projects/test_project_add_duplicate.py