Site-kit-wp: Make sure plugin menu doesn't disappear from left nav

[marrrmarrr]

Bug Description

Currently, some plugins override Site Kit's position in the left nav menu and it completely disappears, making it impossible for users to access the dashboard at all (e.g. #392 ).
Let's make sure the Site Kit entry does not disappear in these cases.

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Acceptance criteria

• When other plugins are overriding menu positioning, Site Kit nav entry doesn't disappear.

Implementation Brief

• Remove the specific $position value (of 3) from Site Kit's add_menu_page() call (so that it's theoretically appended somewhere at the end).
• In the Screens class, implement a similar mechanism like Jetpack uses to ensure the menu item is not overridden.
  
  
  
  • Minor adjustment: We need to check for all slugs that starts with Screens::PREFIX, and they need to be moved to after the index.php item (dashboard).
  
  

Changelog entry

• Ensure Site Kit admin menu item cannot accidentally be overridden by other plugins.

[aaemnnosttv]

Implementation brief looks good, although this part is a bit unclear to me:

We need to check for all slugs that starts with Screens::PREFIX, and they need to be moved to after the index.php item (dashboard).

Wouldn't only the top-level menu item need to be moved in the order?

[felixarntz]

Wouldn't only the top-level menu item need to be moved in the order?

Yes, but this list only includes top-level menu items anyway (and the slug used for it may differ on the available permissions of the current user).

[aaemnnosttv]

Ok, got it. Sounds good 👍

[aaemnnosttv]

@felixarntz code review approved ✅

[felixarntz]

QA passed