Issue #402 - Stopped using specialization attribute from subscriptions.xml

This MR contains backports the changes from https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/commit/f5bb646cde928233f31dca821cdeb4ba8fcd66ec but excluding the ones relating to the font generation script because that one is only part of %Release 2019-1.

Note that we will also need to cherry-pick https://gitlab.com/eyeo/adblockplus/abpui/adblockpluschrome/commit/5ba6686b75f227bd34522b6bdd6d44e55a2b8dde into compate-next-2019-1 for our adblockpluschrome fork.