Feature name format only enforced by frontend

Via the API, you can create a feature with name "My Feature Name" in a project. This validated the FE-implemented validation by adding whitespace, and would also be possible in a project that only allows lower case feature names, violating another validation rule.

We should duplicate all feature name validation to the backend and, where relevant, remove from the FE.

As we implement this, we need to verify that any features that exist with an invalid name can still be updated (e.g. to add tags, or change to server-side only, etc.). It should not be possible to create _new_ features with names that violate the new validation rules.

Acceptance Criteria:

- [ ] All validation logic currently implemented in the FE is reflected in the BE