Verify change of single site admin email address before it changes

[johnbillion]

On a multisite installation it's not possible to change the Email Address setting on a site without first clicking the confirmation link that gets emailed to the new address.

This same behaviour should be implemented on regular single site installations to avoid the address being inadvertently set to an incorrect address.

[MatheusGimenez]

Added verify email on change site admin email in single site installations

[Presskopp]

Related: #39119

[jbpaul17]

Punting to 4.8.1 per today's bug scrub.

[jbpaul17]

Per today's bug scrub, we'll punt this as the focus for 4.8.1 is regressions only.

[johnbillion]

39118.2.diff​ is a refresh plus some fixes that take into account the option whitelisting. I'm happy with this and I think it's good to go.

[johnbillion]

In 41254:

Options, Meta APIs: Require a confirmation link in an email to be clicked when an admin attempts to change the site admin email address.

This adds this previously Multisite-only functionality to single site installations too. This change prevents accidental or erroneous email address changes from potentially locking users out of their site.

Props MatheusGimenez, johnbillion

Fixes #39118

[johnbillion]

In 41255:

Options, Meta APIs: Update the multisite unit tests after [41254], [41164], and [41163].

This moves some more previously Multisite-only tests into the main test suite, and makes small adjustments to their assertions.

See #39118, #16470, #39117