Enqueue editor and block scripts only when necessary

[bobbingwide]

With the current version of the new block editor plugin there have been issues with plugins enqueueing scripts which then cause failures in parts of the admin interface.

Noted so far in issue 4678,

• classic editor fails to load
• Text widget fails to load

These were being addressed in PR 4693 but this has been closed due to technical difficulties.

We need to ensure that we are responding correctly to the action hooks by taking into account the context. The table below shows the scripts we should enqueue in response to the action hooks that are invoked. The ; prefixes indicates that the hook is nested in the hook in the preceding row.
The Y indicates if the hook is invoked. Note that replace_editor is a filter.

Hook	Front end	Block editor	Classic-editor	Admin
wp_head	Y
; wp_enqueue_scripts	Y
; ; enqueue_block_assets	Y - frontend_scripts
replace_editor		Y	Y
; admin_enqueue_scripts		Y
; ; enqueue_block_editor_assets		Y - editor_scripts
admin_enqueue_scripts			Y	Y

This table relies on Gutenberg only calling do_action for these hooks when it's appropriate.
It assumes the fix for 4678 has been implemented.

The Block editor is invoked for Add new ( postnew.php ) and Edit ( post.php )
except when the classic-editor query parameter is set; in which case the Classic Editor is used.

Responding to action hooks is one way that a plugin/theme can specify which scripts to enqueue. Another method is through the register_block API.
Investigate how this works then adjust the next paragraph accordingly.

Gutenberg doesn't really help us to determine whether or not we need to enqueue the frontend scripts ( and styles ). It assumes that they'll be needed.... which probably won't be the case for the majority of front end displays until users start using dynamic blocks wholesale.

[bobbingwide]

We also need to cater for the fact that the Gutenberg plugin may not be installed / activated.

[bobbingwide]

Responding to action hooks is one way that a plugin/theme can specify which scripts to enqueue.
Another method is through the register_block_type API.
Investigate how this works then adjust the next paragraph accordingly.

For the first part of this we'll change the styling of the Contact form block and then alter the way that it's enqueued, using register_block_type.