Add cache busting query argument based on index timestamp.

Ensures that we always request a new LunrJS index when a new repo is built and deployed. Does so by appending the repo timestamp to the query parameters so that it is a different request to previous requests.

See it in action here: http://cache-busting-fdroid-website-pserwylo.surge.sh/fdroid-website/ and observe that the network requests for /js/index.json (the LunrJS search index) have the ?timestamp=... query parameter appended, which is the same as the timestamp found in https://f-droid.org/repo/index-v1.jar.