Twenty Seventeen: Post Template block has list-style: disc in editor when front end has no list style

[nkeller15]

Steps to reproduce:

1. Activate Twenty Seventeen Theme
2. Add the Posts List block and choose the Grid pattern
3. Notice how disc list style is applyied to list elements
4. Save and view published page where discs are not visible

[nkeller15]

editor screenshot

[nkeller15]

front end screenshot

[nkeller15]

patch

[sabernhardt]

front end page, with no discs for list items

[sabernhardt]

editor, before applying patch

[sabernhardt]

editor, with the patch

[sabernhardt]

The discs also show in the editor the List view, and the patch removes them from both the Grid and List views (as it is on the front).

Using the :not(.wp-block-gallery, .wp-block-post-template) selector list does not increase the specificity. Because this only changes the editor, I am not concerned about browsers that cannot support a selector list inside :not().

[audrasjb]

Self assigning for final review and commit.

[audrasjb]

In 55913:

Twenty Seventeen: Improve Grid View variation rendering in the editor for the Post List block.

This changeset updates Twenty Seventeen's block editor stylesheet to remove an unwanted disc list style on the "Grid" variation of the Post List block.

Props nkeller15, sabernhardt.
Fixes #58531.