Ability to specify absolute or relative links only in Link fields

In #240, we added Link module to core. When using link fields, I often find that I only want absolute full URLs to be supported, and not relative URLs. Right now all Link fields allow both relative and full URLs. To ensure we get a more predictable input and that fields are used in the way intended by site builders, we should: - Add an option to all Link fields for "Absolute" and "Relative" URLs. - Default to "Absolute" only, as that is the expected format for links. Usually internal linking is done with either a reference field or a menu item.