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 now more general and can be
configured using $compileProvider#imgSrcSanitizationWhitelist and
$compileProvider#aHrefSanitizationWhitelist.

This commit also refactors the linky filter to use $sanitize as a
service instead of directly referencing private functions of $sanitize.

Fixes #3748.