postids are included in block-template-parts & prevents FSE themes from being used across different sites.

[skorasaurus]

Describe the bug
postids are included in html files within the block-template-parts when they are edited within the site-editor and then exported.
As a result, the templates are tied to a particular WordPress installation and prevents themes from being used across different websites.

To reproduce
Steps to reproduce the behavior:

1. Create a full-site theme in Gutenberg (followed steps as of 78eb4ae).
   1a. This includes creating an index.html
   that consists of <!-- wp:template-part {"slug":"header","theme":"myfirsttheme"} /--> <!-- wp:template-part {"slug":"footer","theme":"myfirsttheme"} /-->
2. Make edits to the theme's header and footer within the site-editor GUI.
3. Go to wp-admin/admin.php?page=gutenberg-edit-site and export your theme's templates and overwrite them into the full-site theme that you created locally on your computer in step 1.
4. notice that the the index.html and single.html now contain a postID field
   e.g. <!-- wp:template-part {"postId":21984,"slug":"header","theme":myfirsttheme"} /-->
5. copy the theme's folder to a new wordpress instance; maybe you want to share your theme with a friend or the WordPress Community.
6. within your other site, activate the theme;
7. on your other site, wp-admin/edit.php?post_type=wp_template_part your template parts are not found there :(

Expected behavior
FSE themes would work across different WordPress installations and sites.

Editor version (please complete the following information):

• WordPress version: 5.5.
• the theme was created gutenberg plugin 9.1; the other site was using a newer version of gutenberg, custom built from the main branch, last commit on 2020-10-02.

[youknowriad]

closed in #26268