Don't load PHP classes in WP5+

[mcsf]

Description

See report in Core: https://core.trac.wordpress.org/ticket/45157

Edit: Superseded by #11375.

See diff with no whitespace changes.

After a lot of back-and-forth, I suspect any failure we may see here is due to interactions caused by Gutenberg-dependent plugins. The class_exists guards should be more than enough to allow the plugin to run on both 4.9.8 and 5.0. In any case, we can try to shield the user even more by checking that we're running 4.9.x before loading classes in the plugin.

How has this been tested?

Plugin should work in 4.9 and 5.0. Test server-side interactions, like editing Latest Posts, viewing Latest Posts on the frontend. If you have a plugin using server-side knowledge of blocks, try that too.

Note: Latest Comments seems to be having problems in core regardless of the plugin.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[mcsf]

Perhaps we can reassess core's upgrade framework to see how it can avoid loading plugin classes that could conflict with an upgrade. As for this PR, it's not really useful. cc @pento