[6.x] Add default initial option configuration option for Link fieldtype

[martyf]

This adds a new default_option config option for the Link fieldtype. This allows a new field instance to have a configurable pre-set type - meaning for commonly used types, two less clicks.

The current behaviour is:

• when required, url
• when optional, null (appears as "None")

This behaviour is maintained when no default_option is set.

However, when a default_option is set, the default_option becomes the initial option for the fieldtype instead of the url or null above.

first-child and asset are special cases as they require specific conditions. If one of these is set as the default_option, it only is set as the initialOption in the fieldtype meta if it is actually available for the field.

Tests have been included for this new functionality.