`abp:subscribe` links on adblockplus.me do not work with new uBlock install

[gorhill]

To repro:

• Paste the following text into external lists textarea (this is the default content):

! Examples:
! https://easylist-downloads.adblockplus.org/antiadblockfilters.txt
! https://easylist-downloads.adblockplus.org/fb_annoyances_full.txt
! https://easylist-downloads.adblockplus.org/fb_annoyances_sidebar.txt
! https://easylist-downloads.adblockplus.org/fb_annoyances_newsfeed.txt
! https://easylist-downloads.adblockplus.org/yt_annoyances_full.txt
! https://easylist-downloads.adblockplus.org/yt_annoyances_comments.txt
! https://easylist-downloads.adblockplus.org/yt_annoyances_suggestions.txt
! https://easylist-downloads.adblockplus.org/yt_annoyances_other.txt

• Go to https://facebook.adblockplus.me/en/
• Try to subscribe to any of the lists

After confirmation, the subscription didn't take effect.

This is because uBlock performs a bit too naive a test to find out whether an external list is already subscribed to.