Theme directory search handling breaks global search

[ryelle]

The global search bar is broken on Theme Directory, nothing happens when you try to search.

To reproduce:

1. Visit ​https://wordpress.org/themes/ or a single theme like ​https://wordpress.org/themes/twentytwentythree/
2. Click the search icon in the global header (not the theme search), the search opens
3. Type any search terms
4. Hit enter or click the search icon button
5. Nothing happens

It should direct you to the global search page, wordpress.org/search, with your results. Instead, there's a JS error that TypeError: a.closest(...) is null

I think the theme's JS handling is accidentally grabbing the global search, but the different markup causes the JS error that prevents the form from submitting.

Props to @nielslange for the initial report.

[coffee2code]

I know the cause of this. Fix incoming.

[coffee2code]

In 12604:

Theme Directory: Limit the categorization JS to its respective form.

Prevents a JS error and interfering with other JS.

Props nielslange. ryelle, coffee2code.
Fixes #7013.