Fixes broken form for feature flags

What does this MR do?

The edit and remove operations in the ff form were updating the scopes based on the array's index. However, since the table is using a filtered version of the scopes, the index did not match.

This commit creates an internal id for the new scopes, the ones sent from the API already have one. It now uses the id key to update the existing scopes.

Before sending the data to the server, it removes the internal id key.

What are the relevant issue numbers?

Closes #10233 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer