Add UI to notify users that social blocking was done

[bbondy]

This is Step 3 of the plan outlined in this issue:
#3475
In the section "Summary of the plan"

Step 2 of this plan was here:
#3489

Step 3: Add a UI which allows us to block by default but inform the user in case they want to enable the functionality.

To do those options I think a good way will be via an extension to AdBlockPlus filter syntax for tagging filter rules:
$tags=embedded-tweets would tag a rule as being related to the embedded tweets option.
When we do matching, if the option for embedded tweets is off, we'd ignore any filters with the tag embedded-tweets. Clients would specify matching options to use, and AdBlockClient::matches would ignore the things that are specified.