Initialize slug from the store when in block editor

[abotteram]

Context

• Fixes a bug where the slug would not be shown in the snippet editor.

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where the slug would not be shown in the snippet editor.

Relevant technical choices:

Test instructions

This PR can be tested by following these steps:

• Create a new post.
• Refresh the editor page and make sure the slug is filled in in the snippet editor.
• Update the slug through the snippet editor and see that the slug is updated in the document settings.
• Update the slug through the document settings and see that the slug is updated in the snippet editor.
• Repeat these steps in the classic editor.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #P2-250

[hansjovis]

Acceptance test: 🚫

getEditedPostSlug is a new selector method introduced in WordPress 5.5. The test fails on 5.4.2.
More specifically: the method has been introduced in Gutenberg 7.8.

[hansjovis]

I am investigating if changing it to a check on the existence of editorSelectors.getEditedPostSlug might work.

[hansjovis]

CR: 👍

[hansjovis]

Acceptance test: ✅

• For published posts, reloading the page correctly fills in the slug (for 5.4, 5.5, in Gutenberg and the Classic Editor).
• However, for draft posts this is not the case.
  • Though, this seems to be an existing bug, since it also happens on master. I am not sure what the behavior should be for draft posts, I will check this next monday. So I am going to merge this.