In this patch, we move from a model where URL classification is opt-in
(by specifying LOAD_CLASSIFIER_URI) to a model where it is enforced by
default unless a channel opts out or is deemed to be a critical channel
based on simple heuristics.
The heuristics exempt a channel from classification if it is triggered
by system with an exception when it is a top level load.
To ensure critical channels are never classified, we also exempt
channels which are flagged as "BeConservative" (implemented in bug 1321783).
Another load flag LOAD_BYPASS_URL_CLASSIFIER is also introduced
for the same reason.
Differential Revision: https://phabricator.services.mozilla.com/D22110
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extra : moz-landing-system : lando
d97a1a7436