Filter out Drip's email tracker from URLs

[fmarier]

Drip uses the __s query string parameter to link user visits to a website back to their email address.

Here's an abridged example:

$ curl -i https://t.dripemail2.com/c/eyJhY2NvdW50X2lkIjoiNTQzOTc1OSIs...WxwLTEwMS8_X19zPXpjcmFhdHNqYnRlc3NzbjJraXlrIn0
HTTP/2 307 
date: Thu, 02 Apr 2020 00:02:48 GMT
content-length: 0
location: https://skipperotto.com/kelp-101/?__s=zcra...2kiyk

The long parameter in the first URL is a base64 encoding of the following information:

{
    "account_id": "5439759",
    "delivery_id": "9ne...gdv",
    "url":"https://skipperotto.com/kelp-101/"
}

According to the Drip documentation, removing this parameter "disables Drip’s ability to identify new people and track already identified people."

[fmarier]

Since this is a pretty generic-looking parameter name, we might want to restrict the query filter further before adding this to the list:

• exempt same-site requests from the query filter (Exempt intra-site requests from the query string filter #9020)
• require a minimum value length (e.g. 6 characters) before stripping
• check that Shields is enabled (suggested on Prevent tracking based on link decoration via query string or fragment #4239 (comment))

[stephendonner]

Verified FIXED using the testplan from brave/brave-core#7968 with

Brave	1.22.39 Chromium: 89.0.4389.58 (Official Build) nightly (x86_64)
Revision	1a139f28ecc27719439e37c6b1533cee999cb802-refs/branch-heads/4389@{#1134}
OS	macOS Version 11.2.1 (Build 20D74)

1. Open https://brave.com/?fbclid=1234 in a new tab.
2. Check that the fbclid query parameter is missing from the URL bar.
   
3. Turn Shields OFF for brave.com.
4. Open https://brave.com/?fbclid=5678 in a new tab.
5. Check that the fbclid query parameter is present in the URL bar.
   
6. Turn Shields ON for brave.com.
7. Open https://brave.com/?__s=abcd in a new tab.
8. Check that the __s query parameter is missing from the URL bar.
   

[srirambv]

Verification passed on OnePlus 6T with Android 10 running 1.22.54 x64 beta build

• Verified test plan from Tie query filter to Shields setting and add Drip tracker brave-core#7968

Android.ARM.mp4

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Verification passed on Samsung Tab A with Android 10 running 1.22.54 x64 beta build

• Verified test plan from Tie query filter to Shields setting and add Drip tracker brave-core#7968

Android.Tab.mp4