Smart Linking: Don't auto-remove existing links

[acicovic]

Is your feature request related to a problem?

When we use Smart Linking, the existing links of the post or selected block get removed.

Describe the solution you'd like

I'd assume that in most cases, we do want to keep existing links added by the user, and only regenerate the links injected by Smart Linking. However, the user might wish to keep some of the links injected by Smart Linking, so giving the user granular control seems to make the most sense here.

[acicovic]

This should be fixed per #2436. Closing.