Add validations and errors handling for StaticSiteEditor

What does this MR do?

Contributes to #212558 (closed)

• Add permissions check for a user role
• Add validations for
• Expose is_supported_content boolean field to frontend

is_supported_content shows if there are no validation errors detected by backend.

Validation error messages are not used for now. Frontend will show a general error message for all validation failures.

Previous MRs:

1. Add StaticSiteEditor endpoint - !28689 (merged)
2. Provide payload to initialize StaticSiteEditor - !29058 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team