Sanitize the add-on blocklist update URL

The default value of the extensions.blocklist.url preference is

!https://blocklist.addons.mozilla.org/blocklist/3/%APP_ID%/%APP_VERSION%/%PRODUCT%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/%PING_COUNT%/%TOTAL_PING_COUNT%/%DAYS_SINCE_LAST_PING%/

and sends detailed information about the operating system to Mozilla.

However, Mozilla's list of blocked add-ons and certificates is not OS specific, and updates just need

!https://blocklist.addons.mozilla.org/blocklist/3/%APP_ID%/%APP_VERSION%/

so that should be the default value of extensions.blocklist.url in Tor Browser.