Open source themes info API endpoint

[ocean90]

When the theme directory was rewritten the info endpoint was also rewritten. The API is currently fully documented and pretty straightforward.

@obenland and @dd32 have no objections for this task. We probably only want to remove the "placeholder sites" first.

____themes
| |____info
| | |____1.1
| | | |____index.php
| | |____1.0
| | | |____index.php
| | | |____class-themes-api.php

[dd32]

In 2755:

Themes Directory: Open-source the pre_get_posts logic used by the Themes Directory & add logic to handle the domain not matching the home_url.

This allows us to remove the 'placeholder' sites we use for the themes directory on internationalized sites (xx.wordpress.org/themes), and makes the query logic more transparent.

See #1630 (which this is the first part for)
See #1618 (Some work will still be needed to correctly trim the description)

[obenland]

@dd32 Would you mind pairing up with me on this one?

[dd32]

@obenland Sure, however looking at the code, there's a few things that need cleaning up before we can open-source some of the API code. Although most of it is portability to other systems and not making assumptions that it'll be run on WordPress.org.

[dd32]

In 6418:

API: Open-source api.wordpress.org/themes/info/ and api.wordpress.org/themes/theme-directory/.

See #1630

[dd32]

In 6419:

Themes: Move the Themes_API query class into the Theme Directory plugin.

See #1630

[dd32]

In 6462:

Theme Directory: API: Move the Themes_API class into the plugin, making api.wordpress.org/themes/info/ a wrapper for it.

See #1630

[dd32]

This is now completed.