Update themes upload page

[dingo_d]

Current page ​https://wordpress.org/themes/upload/ is a mix of content from the admin and page template.

This makes the site non-translatable, which can be a problem as presented here ​https://sv.wordpress.org/themes/upload/

The patch I've created will add all the content in the page template.

Another bug this fixes is the fact that the wporg-themes-upload shortcode echoed the content out instead of returning it, which caused errors on page update from the admin (plus the shortcode was literally present in the admin area).

I'll submit a patch that covers page-upload.php in the wporg-themes theme and upload.php in the theme-directory plugin.

When this will be applied, the content on the page should be removed (aside from the shortcode which should remain).

[dd32]

In 9345:

Theme Directory: Convert the uploads page into a real shortcode (That returns content) and hard-code the strings on the upload page to allow translation.

Props dingo_d.
Fixes #4839.

[tobifjellner]

There was a typo in a translators comment: Theme Handbook Requied Review items.

[tobifjellner]

One more thing: It's a bit strange to hardcode the pseudo filename themename.zip in the text. It would be better to allow it to be translated.

[dd32]

In 9351:

Themes: Correct a typo in a translator comment.

Props tobifjellner.
See #4839.

[dd32]

Replying to tobifjellner:

There was a typo in a translators comment: Theme Handbook Requied Review items.

..I really need to get my Mac keyboard replaced.. (The R key doesn't work too well sometimes).

Fixed. Thanks!

One more thing: It's a bit strange to hardcode the pseudo filename themename.zip in the text. It would be better to allow it to be translated.

While I agree, I don't trust it to be translated properly/well and I don't think there's a massive benefit to translating it, as most locales will likely translate it as-is.