Add pattern validation to slug fields in dimension editor

[japsu]

Currently this is the validator used in backend:

https://github.com/con2/kompassi/blob/69c3418ab4da651f6eab9c2890c60076b29f7623/backend/core/utils/model_utils.py#L9-L12

However, this is not validated on the forms, causing a GraphQL error and #399.

Add a similar pattern validator in the frontend to the inputs that convey slugs.