fix($location): always resolve relative links in html5mode to `<base>` url

BREAKING CHANGE (since 1.2.0 and 1.3.0-beta.1):

Angular now requires a `<base>` tag when html5 mode of `$location` is enabled. Reasoning:
Using html5 mode without a `<base href="...">` tag makes relative links for images, links, ...
relative to the current url if the browser supports
the history API. However, if the browser does not support the history API Angular falls back to using the `#`,
and then all links would be broken.


BREAKING CHANGE (since 1.2.17 and 1.3.0-beta.10):

In html5 mode without a `<base>` tag on older browser that don't support the history API
relative paths were adding up. E.g. clicking on `<a href="page1">` and then on `<a href="page2">`
would produce `$location.path()==='/page1/page2'. The code that introduced this behavior was removed
and Angular now also requires a `<base>` tag to be present when using html5 mode.

Closes #8172

Yes, this is a breaking change. But in html5 mode you should not use those kinds of urls, right? They are used in plain hashbang mode (without html5Mode turned on)