Creates a 'prePublish' action hook that is triggered
right before a post is published.

This reverts commit 81696afec2bd3c37e78d944b73c7978b06431ad2.
Using action hooks and an extra request was deemed too much kludge
just to differentiate publishing from Gutenberg. Another iteration
is incoming...

As a method for differentiating requests from Gutenberg editor,
a header keyed 'X-WP-Source' is being added and set to 'Gutenberg'.

Adding slots for pre-publish sidebar and post-publish sidebar to be used
with `registerPlugin` for plugins to add content to the bottom of these
sidebars. The current plan is to use the post-publish sidebar will be used
for Jetpack's Publicize feature.

Cleaning up some leading and trailing whitespace issues.

Fixing typos and clearing out unecessary DocBlock entries.

Removing obsolete curly brackets around PluginPostPublishPanel.Slot and PluginPrePublishPanel.Slot components.

Now passing pre/post publish plugin slot using new prop and children prop
instead of referencing it directly from within editor.

props @youknowriad

Replacing repetitious code with newly created createSlotFill factory function
for PluginPrePublishPanel and PluginPostPublishPanel slots.

props @gziolo