Revert #3218 and deprecate accessing global paths from Handlebars

**In Progress**

As discussed in #3218, this PR reverts the addition of the `ember-handlebars-caps-lookup` feature flag and adds a deprecation warning when global paths are accessed from a Handlebars template.

I'm trying to fix the original inconsistency that was reported in #3098 (between `{{Foo}}` and `{{#each Foo}}`) by changing bindings to look up values on the given context before `Ember.lookup`. That part is incomplete and I'm not really sure what to change to get the rest of those tests to pass. Would love to be pointed in the right direction there.

I'm also not sure if it's possible to add a deprecation notice in the `{{#each Foo}}` case since the binding doesn't know whether it's being created from a template or not.

cc @ebryn