Add partner and community themes to the custom theme-install.php

[scruffian]

What

When searching for themes in theme.php users should also see community themes (from WordPress.org) and partner themes.

Why

This is what happens in the Calypso showcase, so we should match that functionality.

How

There is probably already an API that Calypso uses for this, so we should be able to reuse that.

[github-actions]

This issue could use some more labels, to help prioritize and categorize our work. Could you please add at least a [Type], a [Feature], and a [Pri] label?

[ryelle]

In 170169-ghe-Automattic/wpcom/, partner themes are already returned, but not labelled or processed correctly so they show up broken.

Community themes are not returned yet. IIRC, community themes should only return on search, so that someone searching for a specific theme can find it.

There is probably already an API that Calypso uses for this, so we should be able to reuse that.

Calypso just uses the wporg API http://api.wordpress.org/themes/info/1.1/… and seems to merge the themes into the result set.

[ryelle]

Update on 170169-ghe-Automattic/wpcom/ — Partner and community themes are included in the search result set now, and displayed like the other themes (not labelled yet, I left that for #98655). Partner themes show up integrated into in the result set, while community themes only appear first if it's a match for the search term, otherwise they're appended to the end of the search results (up to 40 community themes, otherwise there are too many results).