previously we were doing all kinds of checks to see if we should rewrite the url or not and we
were missing many scenarios. not any more.

with this change, we rewrite the url unless:
- the href is not set
- link has target attribute
- the absolute url of the link doesn't match the absolute prefix for all urls in our app

This also means that ng-ext-link attribute which we previously used to distinguish external
links from app links is not necessary any more. apps can just set target=_self to prevent
rewriting.

BREAKING CHANGE: ng-ext-link directive was removed because it's unnecessary

apps that relied on ng-ext-link should simply replace it with target=_self

It turns out that listening only on "blur" event is not sufficient in many scenarios,
especially when you use form validation you always had to use ngModelnstant
e.g. if you want to disable a button based on valid/invalid form.

The feedback we got from our apps as well as external apps is that the
ngModelInstant should be the default.

In the future we might provide alternative ways of suppressing updates
on each key stroke, but it's not going to be the default behavior.

Apps already using the ngModelInstant can safely remove it from their
templates. Input fields without ngModelInstant directive will start propagating
the input changes into the model on each key stroke.

Now makes sense.

Explained why you'd want to manually bootstrap, added contrasting
example for automatic vs manual methods.

Added examples, explained the reasons why you initialize the whole app
or parts of the page.