[META] Support WYSIWYG embedding of media entities

Problem/Motivation

Core currently provides support for embedding of images. It is currently impossible to embed a video or a document. Also, when #2801277: [META] Support remote media assets as first-class citizens by adding Media Entity module in core → lands we'll need to embed media entities rather than files.

Contrib already has a solution for this. Entity embed is a module that supports embedding of any renderable entity and supports extensive display configuration.

Proposed resolution

Move parts of Entity embed → into core (simplified text filter) and handle embedding through it. Entity embed display plugins, embed button config entities, and other related API surface will stay in contrib. The simplified text filter will only support embedding media entities; people wishing to embed other types of entities will need to use contrib. Existing WYSIWYG functionality would remain the same.

Remaining tasks

In order:

1. ✅
2. ✅
3. ✅
4. ✅
5. ✅
6. ✅
7. SHOULD-HAVE: 🐛 [regression] Restore styling for embedded media edit button in Seven theme Postponed: needs info
8. SHOULD-HAVE: 📌 Constrain the width of aligned images, media, blockquotes etc to a max of 75% Needs work
9. SHOULD-HAVE #3073901: Determine an upgrade path from CKEditor image button to media library →

Additional required bug fixes:

• #3067116: text_summary() returns malformed (not normalized) HTML for basic_html and other formats that use filter_html instead of filter_htmlcorrector →
• 🐛 text_summary() returns a plain string, even if passed a MarkupInterface object Needs work

Additional related and follow-up issues

• #3100066: "Convert line breaks into HTML" filter should exclude tag →
• #3100470: EditorMediaDialog triggers an "undefined index" notice for data-view-mode →
• #3085436: Center alignment of media embeds doesn't work in Bartik →
• ✨ Add a button to remove an embedded media item from the editor Needs work
• 📌 Improve the UX of captioning in CKEditor Closed: outdated
• #3074867: Remove role attribute from figure tags generated by filter_caption →
• 🐛 Improve icon colour contrast for WCAG 2.1 Active
• #3068786: Allow customization of missing media indicator returned by MediaEmbed filter →
• #3069525: Refactor to remove drupalunlink event listeners for registered linked widgets →
• #3081303: Allow media embed width constraint to be configurable →
• ✨ [upstream] Allow CKEditor styles for Postponed

User interface changes

TBD

API changes

TBD

Data model changes

TBD