Only allow adding filter lists loaded via HTTPS

Background / User story

See https://gitlab.com/eyeo/adblockplus/adblockpluscore/issues/5.

What to change

• Design: N/A
• Research: N/A
• Spec: eyeo/specs/spec!283
• Development: Reuse existing URL validation to prevent users from adding a filter list whose URL uses any other protocol than HTTPS.

Hints for testers

Custom filter list dialog

• No error message is expected to be shown as long as the custom filter list URL input field is empty.
• In addition to https: URLs, Core also allows data: URIs so users should be able to enter either one and only receive an error message when attempting to add a URL with a different protocol than those.
• Otherwise invalid URLs should continue to show existing error message.

Filter list table

• Filter list download error messages shown in filter list table should correspond to the ones mentioned in the spec.
• Filter lists using https: or data: should be shown the new error message. If the URL is invalid in other ways, the existing error message should be shown.

Hints for translators

• Added the following strings:
  • options_filterList_lastDownload_invalidURLProtocol
  • options_dialog_import_subscription_location_error_protocol
• Updated the following strings:
  • options_filterList_lastDownload_invalidURL
  • options_filterList_lastDownload_connectionError
  • options_filterList_lastDownload_invalidData

Integration notes

N/A