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 those relative links would be broken.

The `<base>` tag is also needed when a deep url is loaded from the server, e.g. `http://server/some/page/url`.
In that case, Angular needs to decide which part of the url is the base of the application, and which part
is path inside of the application.

To summarize: Now all relative links are always relative to the `<base>` tag.

Exception (also a breaking change):
Link tags whose `href` attribute starts with a `#` will only change the hash of the url, but nothing else
(e.g. `<a href="#someAnchor">`). This is to make it easy to scroll to anchors inside a document.

Related to #6162
Closes #8492

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, #8233