Site Settings: Mark fields as changed when changing in wrapSettingsForm

[tyxla]

This PR fixes #10670, where it was reported by @drw158 that the Settings: Discussion form does not alert users when changing to another Settings tab with unsaved settings.

This appears to have been introduced in #9899, which reduxified the Discussion form.

The solution proposed in this PR is to mark each field as changed when handling a change in any of the fields.

To test:

• Checkout this branch
• Go to /settings/discussion/$site, where $site is one of your sites.
• Change any setting, do not click "Save Changes".
• Switch to another tab and verify you're presented with an alert that prevents you from losing unsaved changes.

/cc

• @drw158 as the initial reporter of the issue
• @youknowriad because he worked on the Reduxification of the discussion form (Update/form discussion settings #9899)
• @ryelle because she also worked on the Discussion tab recently.

[matticbot]

Test live: https://calypso.live/?branch=fix/wrap-settings-form-markchanged

[youknowriad]

I made a similar change here #10691 but yours is more complete. I only updated the handleToggle callback because most of the time when we use onChangeField or handleRadio, we're relying on regular HTML inputs which should trigger the onChange callback of the form (But I guess these methods could be called in other places, so it's fine to keep these calls to markChanged.

Though, I don't think this was introduced by the reduxification itself. I guess when we were using FormCheckbox the form onChange where being triggered since we were using regular HTML inputs, but it's not the case anymore with toggles.

BTW, This is testing well! 👍

[tyxla]

Thank you for the review, 🐑 ing.