The source of alt text in embedded image media is not clear

Problem/Motivation

When an author embeds image media in the wysiwyg editor, they can choose to:

1. accept the default text alternative previously stored with the media entity, or
2. override the alt text with an instance-specific alt text,

If they accept the default alt text (because it is appropriate for the context) this isn't explicitly recorded; i.e. nothing is stored in the editor markup.

BUT if the default alt text stored with the media entity changes at a later date, then the new alt text propagates to the all existing embed instances which haven't overridden it.

The problem is that the original alt text may have been accepted because is was appropriate to the context, but the new alt text might not be. The upshot is that there may be unintended side effects, where content effectively changes it's meaning.

This is almost in the realm of accidental data loss.

Proposed resolution

When authors embed a media image, and accept the default ALT text from the media entity, explicitly store it in the <drupal-media alt="whatever" markup.

Remaining tasks

TBD.

User interface changes

TBD.

API changes

TBD.

Data model changes

None?