Revert #3218, replace #4358. Deprecate global access from templates

Replaces #4358. These deprecations are narrow in scope, and only impact `{{Global}}` and `{{#each Global}}`. They do not deprecate any other global access in templates. Deprecating all global access from templates would a) need to be done helper-by-helper or b) would impact the `get` and binding code at a lower level.

@xtian I'm rewritten your first commit but preserved it here.

@ebryn yo.