Why:
* Slug must be unique among:
1. Budget slug: among other budgets
2. Group slug: among other groups from its budget
3. Heading slug: among other headings from all the groups from its budget
How:
* Adding checks for all possible scenarios on each model specs
Why:
* Those classes have a slug attribute that needs to be filled
How:
* Just adding the shared example to Budget model spec, and creating the model spec file for Heading and Group to include it as well
