fix(form-builder): improve validation of slugs with namespaces

[anehx]

• Fix display of namespaces / prefixes on slugs
• Convert manual slug uniqueness validation to proper validator
• Validate option slugs with the prefix of the question and check for uniqueness as well

Fixes #1264 Closes #1283

Before	After
please note that the namespace wasn't even included in the initial validation but only when saved