Onboarding: Set standard Jetpack settings

[laurelfulford]

Is your feature request related to a problem? Please describe.
When setting up a site, we manually enable image optimization (for cropping) and the share buttons in Jetpack.

Describe the solution you'd like
Since the settings for the above are the same to start on every site, it would be very helpful if they could be enabled automatically when the wizard is run. Using the UI, these options can only be enabled once Jetpack is connected to WP.com:

Image optimization:
Under Jetpack > Settings > Performance, turning on 'Enable site accelerator' and 'Speed up image load times' (typically enabled automatically in the UI when you turn on 'Enable site accelerator'). This is required to get the image cropping working correctly.

Share buttons:

• Under Jetpack > Settings > Sharing, enable "Add sharing buttons to your posts and pages".
• Once the above is enabled under WP Admin > Settings > Sharing, switch the button style to "Icon only" (the option to customize the buttons from the Jetpack screen brings you to WP.com, so it doesn't work if you're not logged into the account that's connecting the site). This change makes sure the buttons on AMP and non-AMP posts look the same.