Jetpack Settings: Improve handling of all plan & Akismet state cases in Antispam card

[tyxla]

We should be handling all possible cases with plans and Akismet state in the Antispam card:

• Free plan, Akismet installed and activated
• Free plan, Akismet installed but not activated
• Free plan, Akismet not installed
• Free plan, Akismet installed and activated, key stored (probably quite an edge case)
• Paid plan, Akismet installed and activated
• Paid plan, Akismet installed and activated, but no key stored? (probably an edge case)
• Paid plan, Akismet installed but not activated
• Paid plan, Akismet not installed

Here is a detailed list of what each case should do:

#14654 (comment)

More worthy improvements to the card are suggested here: #16796 (comment) and here: #16796 (comment).

[tyxla]

Related: #18257

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