Settings: Change disabled primary buttons to secondary buttons

**Update:** After some chat in the comments, we decided the best path forward would be to hide the "save settings" buttons completely, until an action is performed that requires saving. Then we'll display the buttons.

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Scattered throughout our settings within the plugin we have a bunch of disabled primary buttons that read "save settings." These buttons are disabled until the settings of their respective cards are changed, and then they become active for the customer to save the settings.

To clean things up, I'm proposing that we remove `is-primary` until the buttons become enabled. They have the potential to distract customers and shouldn't be primary until they are active.

### Current settings areas
*(Performance and Sharing areas are fine as-is)*


### Proposed changes to save settings buttons


### Diagram of all targeted buttons

On the surface it seems fairly simple. Remove `is-primary` from all the `disabled` buttons. When `disabled` is removed, we should apply `is-primary`. Thoughts?

Also, under Writing > Composing there appears to be a save button in an area where only toggles exist. If that's the case, I think we can safely remove it from that particular card.

cc @Automattic/jetpack-design