Jetpack Scan: add message for VaultPress users that have the security feature

[v18]

When adding Jetpack Scan for sites that already have VaultPress with a plan that includes the security feature, the SSH credentials are silently accepted.

Instead we should detect the presence of a VaultPress plan with the security feature, and let the users know that they already have VaultPress scanning, and that they cannot also activate Jetpack Scan.

Right now, the only VaultPress plan that has scanning is the Security Bundle.

Steps to reproduce:

1. Add the VaultPress Security Bundle plan to a site
2. Install the VaultPress plugin on the site
3. See the 'Activate Scan' button for Jetpack Scan for this site

[stale]

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