Don't require theme names to follow WordPress.org theme directory rules

I like that the plugin checks if the theme name follows the requirements on the WordPress.org theme directory, but I don't think that themes should be required to follow those rules before a clone/child/blank theme can be created.

The reason is that there's no guarantee that the user intends to submit the theme to WordPress.org. He/she could just as easily be using the plugin to create a theme for personal use, or for a client. In that case, there's no need to make sure that the theme name doesn't contain the word "theme", or that the chosen name is available on the directory.

(In the image above, there's also no context for why the theme can't contain the word "theme".)

Instead of preventing exports when the theme name breaks the WordPress.org theme directory rules, I'd suggest that the plugin informs the user that he/she won't be able to submit the theme to WordPress.org with the chosen name.

Related: #496. If a "Not for release" checkbox like the one described in that issue is added, the name validation against the theme directory rules could be disabled when the checkbox is checked.

Review the existing theme-name validation and export flow described in the issue, then read related issue #496 for the proposed “Not for release” option. Done means personal or client themes can be exported without WordPress.org naming restrictions, while users are informed that the chosen name cannot be submitted to WordPress.org.