Preview link clicked - save a revision

[slaFFik]

If anything was changed before the manual saving (previous version of content) and Preview link click - save content as a revision and then open a post preview (with updated data).

This will give ability to actually save a draft somehow (see related #1672) and preview the latest version of content after the last saved copy and what is currently available in content area.

[westonruter]

We should also take this as an opportunity to ensure that postmeta are included in the revision. See also https://core.trac.wordpress.org/ticket/20564 but alternatively as I'm proposing in #1516 (comment) we could also use the a Customizer changeset to preview changes to posts and postmeta, which would also help
address the concern about an explosion in the number of postmeta fields being inserted.

[mtias]

Saving when clicking preview has been implemented.