Mixed content warnings on theme pages cause preview to redirect to new page

[tollmanz]

On theme pages, screenshots should be clickable, which cause thickbox to load with a theme preview. This is broken due to the necessary assets that control this behavior being blocked in Chrome due to being served via HTTP instead of HTTPS.

To reproduce:

1. Open a theme page in Chrome (e.g., ​https://wordpress.org/themes/make)
2. Open Dev Tools
3. Open the Console
4. Notice the following warning:

[blocked] The page at 'https://wordpress.org/themes/make' was loaded over HTTPS, but ran insecure content from 'http://wordpress.org/bb-theme/themes/thickbox/thickbox.css': this content should also be loaded over HTTPS.
 make:1
[blocked] The page at 'https://wordpress.org/themes/make' was loaded over HTTPS, but ran insecure content from 'http://wordpress.org/bb-theme/themes/thickbox/thickbox.js': this content should also be loaded over HTTPS.
 make:1
[blocked] The page at 'https://wordpress.org/themes/make' was loaded over HTTPS, but ran insecure content from 'http://wordpress.org/bb-theme/themes/thickbox/theme-preview.js': this content should also be loaded over HTTPS.

5. Click the screenshot
6. Notice the preview opens in a new page instead of in the thickbox

[Otto42]

This is caused by a redirection issue from when we moved the theme around.

It can be fixed, but doing so breaks the preview frame because the wp-themes.com domain (where we have the previewer) has an incorrect SSL certificate. We'll need to fix that before fixing this.

[Otto42]

Related: #344

[Otto42]

Duplicate of #344.