Allow modifying settings visibility globally

Problem/Motivation

Since there's some accessibility issues on Settings button 🐛 Settings Button UX Improvements Active and some sites might have a lot of different view modes, it would be nice to control which settings elements are rendered on same_page_preview_form_select (defined on PreviewControlsForm).

Steps to reproduce

Proposed resolution

Add two options to admin/config/content/same-page-preview
1. Checkbox: "Show Settings button" on preview. Enabled by default.
2. Checboxes: "Allow following view modes": and list of view modes used by node entities.
If only one view mode is selected, it probably shouldn't show the select element on same_page_preview_form_select

Remaining tasks

User interface changes

API changes

Data model changes