Make Snippet edit fields v-model aware

What does this MR do?

In anticipation of wrapping the whole Snippet edit form into Vue component with GraphQL connection, the components, constituting the form are tuned to support v-model to make communication with data gathering easier on the top level of the form component.

This is part of a larger effort of Refactor Snippet Edit form actions to Vue . The feature is not really click-testable atm as the main form component will be done on the next step only.

Since this MR doesn't introduce any new user-facing functionality, there's not CHANGELOG entry.

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

References #207463 (closed)