Replace AdBlock Plus with uBlock Origin

Originally created by @drizzt on #9833 (Redmine)

The discussion at https://mailman.boum.org/pipermail/tails-dev/2016-January/010146.html and https://mailman.boum.org/pipermail/tails-dev/2016-February/010163.html sums up a few good reasons to do the switch.

To start with, we should not use uBlock Origin’s features (e.g. tracker blocking) others than ad blocking. One can configure it this way apparently: https://mailman.boum.org/pipermail/tails-dev/2016-February/010191.html and https://mailman.boum.org/pipermail/tails-dev/2016-February/010201.html

Feature Branch: spriver:feature/9833-replace-adblock-with-ublock

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