Remove the "Featured" tab in the Add Themes screen

[kjellr]

Followup to ​5044 (Meta) / ​9522 (Commit)

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The "Featured" tab of the theme repository is the first page that users visit when looking for themes. As such, it should ideally present them with the best of the best WordPress themes. Instead, the featured list is just randomly generated. This does not quite fulfill the screen's purpose.

There have been discussions around replacing this with some sort of actual curated list, or an algorithmically chosen set based on how new and popular a theme is (as proposed in ​3750 (Meta) for example).

Until consensus arrives on an approach for that, we should disable the featured tab.

As noted above, this the ​Meta ticket for this has already been completed, and this change just needs to be carried through to the Add Themes page.

[celloexpressions]

The theme browser in the customizer prototypes an alternative interface that prioritizes filtering and search over the older tabbed approach, defaulting to latest themes. The featured, popular, latest, and favorites tabs do not exist on the premise that they don't actually help users. I don't know if any stats are available to validate that this approach is better, but it's been in core since WordPress 4.9.

Rather than only removing the featured tab I proposed syncing the customizer interface into wp-admin, or even merging these into the single newer interface in the customizer. The customizer integrates theme browsing into the live preview workflow more seamlessly than wp-admin, using live site previews instead of the .org preview.

Removes the link to the featured tab on the Add themes screen.
Sets the popular tab as the default.

Trac ticket: https://core.trac.wordpress.org/ticket/49487

(The changes in this PR needs to be built befor testing)

[SergeyBiryukov]

In 50813:

Themes: Remove the "Featured" tab on Add Themes screen.

Themes displayed in this tab were actually random, and have not been selected to be featured to new WordPress users.

The tab had already been removed from the Theme Directory, and with this change, the options in the admin area and in the directory will match again.

The "Popular" tab is now displayed instead as the default view.

Props poena, kjellr, celloexpressions.
Fixes #49487. See ​#meta5044.

[milana_cap]

Being label for DevHub, needs-docs doesn't really apply to this ticket as we are not mentioning Administration Screens anywhere in Theme Handbook.