Use Stylesheet instead of Template to key theme data when doing theme update check

[josephscott]

When sending theme data during the theme update check using the 'Template' name to key the data is causing problems for parent/child themes. Because they both have the same 'Template' value the child would over write the parent data, basically causing the parent theme to be skipped when doing the theme update check.

So instead of Template we should key the data from the Stylesheet value, which isn't shared in parent/child themes.

[ryan]

(In [11424]) Use Stylesheet instead of Template to key theme data when doing theme update check. Props josephscott. fixes #9897

[Denis-de-Bernardy]

Just curious... in addition to this, shouldn't we enforce that the parent theme is up to date before allowing the child theme to update? Or better yet, update both at the time?