Themes: Retain filters and query string when activating

[ockham]

Previously, when activating a theme in multi-site mode, you'd be taken to the showcase's single-site mode for the site you'd select -- but any filters and search strings would be lost.

This PR retains them. It does so by passing on the full pathName from the controller, and using that to build the redirect target.

To test:

• Verify that the above is true.
• Make sure that activation still works from the theme sheet.

(This started as an attempted fix for #13506 -- however, that issue isn't fixed by this PR.)

[matticbot]

Test live: https://calypso.live/themes?branch=update/themes-site-selector-modal-path

[folletto]

Tested on Chrome:

1. Activated from All Sites with query strings ✅
2. Activated from Single Site with query strings ✅
3. Activated from theme sheet with query strings ✅

To be clear, in (3) above the strings aren't preserved, I was just checking if it still worked.