Add proxy for Adzerk click URLs

[diox]

Like #7967 but for click URLs.

[diox]

Per #13841 it will likely need to behave like this:

• Accept POST with the URL somewhere in post data
• Request that URL via GET
• Don't need to return anything (client won't follow that redirect, the response can be discarded)

[ioanarusiczki]

@eviljeff
I made a POST request on https://addons-dev.allizom.org/api/v4/shelves/sponsored/click/. I tested it for all 6 add-ons returned by GET https://addons-dev.allizom.org/api/v4/shelves/sponsored/ and the response is a 202 Accepted for processing status.

The click_data generated expires after one hour -> I get a 400 Bad request with "Bad click_data: Signature age 4777.010764360428 > 3600 seconds" which is the expected in this case. The same happens with impression_data after 60 seconds (verified in #7967)