Prevent adding non-HTTPS filter lists via subscribe links

Background / User story

We stopped downloading filter lists that are served over non-HTTPS in Adblock Plus 3.6 and prevent users from adding such filter lists in #414 (closed). However, they can also add filter lists via subscribe links (see spec) and it's unclear how we should handle that.

What to change

Design: (Link to any designs here)
Research: (Link to any research data here)
Spec: https://gitlab.com/eyeo/specs/spec/merge_requests/270 (Relevant section: Adding filter lists via subscribe links (lines 86 to 98))

Hints for testers

None.

Hints for translators

Added the following strings:

options_dialog_invalid_message
options_dialog_invalid_title

Integration notes

N/A

Edited Dec 09, 2019 by Thomas Greiner

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