fix($parse): deprecate promise unwrapping and make it an opt-in

This commit disables promise unwrapping and adds
$parseProvider.unwrapPromises() getter/setter api that allows developers
to turn the feature back on if needed. Promise unwrapping support will
be removed from Angular in the future and this setting only allows for
enabling it during transitional period.

If the unwrapping is enabled, Angular will log a warning about each
expression that unwraps a promise (to reduce the noise, each expression
is logged only onces). To disable this logging use
`$parseProvider.logPromiseWarnings(false)`.

Previously promises found anywhere in the expression during expression
evaluation would evaluate to undefined while unresolved and to the
fulfillment value if fulfilled.

This is a feature that didn't prove to be wildly useful or popular,
primarily because of the dichotomy between data access in templates
(accessed as raw values) and controller code (accessed as promises).

In most code we ended up resolving promises manually in controllers
or automatically via routing and unifying the model access in this way.

Other downsides of automatic promise unwrapping:

- when building components it's often desirable to receive the
  raw promises
- adds complexity and slows down expression evaluation
- makes expression code pre-generation unattractive due to the
  amount of code that needs to be generated
- makes IDE auto-completion and tool support hard
- adds too much magic

BREAKING CHANGE: $parse and templates in general will no longer
automatically unwrap promises. This feature has been deprecated and
if absolutely needed, it can be reenabled during transitional period
via `$parseProvider.unwrapPromises(true)` api.

Closes #4158
Closes #4270

Author: IgorMinar