Allow subscribing to filters lists only on HTTPS pages

Background

We implemented the UI changes to limit users to only add and subscribe to filter lists via HTTPS or data-URIs (see ui#414 and ui#450). That led to a discussion whether subscribing via data-URIs could be abused by being served via HTTP themselves.

Therefore we should think about limiting abp-subscribe links to only HTTPS pages to not only ensure that the filter list itself is coming from a trusted source but also that the page that links to that filter lists can be trusted.

Suggested change

Restrict the match patterns that specify websites allowed to use subscribe links to HTTPS only.