fix($sanitize): Use same whitelist mechanism as $compile does.

`$sanitize` now uses the same mechanism as `$compile` to validate uris.
By this, the validation in `$sanitize` is more general and can be
configured in the same way as the one in `$compile`.

Changes
- Creates the new private service `$$sanitizeUri`.
- Moves related specs from `compileSpec.js` into `sanitizeUriSpec.js`.
- Refactors the `linky` filter to be less dependent on `$sanitize`
  internal functions.

Fixes #3748.