Standardise our GraphQL enum types
## Description of the proposal
Currently, we have 5 types inheriting from `BaseEnum`, but they have some inconsistencies in their implementations.
We should:
- Aim to standardise our GraphQL enum types
- Document the standards in our [GraphQL Developer Styleguide](https://docs.gitlab.com/ee/development/api_graphql_styleguide.html)
- Enforce the standards with new cops
### Standards
The standards are up for debate, but as a proposal:
**1. The values should be uppercase**
Please debate this one! Currently, 4 enums have lowercase values and only 1 (`PipelineStatusEnum`) has uppercase values. However, it may be _more_ correct for GraphQL enums to have uppercase values. But it's our API so we get to decide.
This change would cause the most divergence from what we currently have.
This may not be enforceable by a cop.
**2. The class name should end in `Enum`**
This is already the case across all enums.
**3. The `graphql_name` of enums should not have `Enum` in it**
This is the case for all except `PipelineStatusEnum`, which has no `graphql_name` defined so it identifies using its class name.
- Deprecate existing inconsistencies
- Implement new cop to enforce consistencies
epic