Add config option so more esoteric link types can be removed from list of default link

[maxime-rainville]

By default, all link types are available in a LinkField. If a developer wants to disable a specific Link type, they have to do so for each instance of LinkField.

There could be some situation where this is not advisable. e.g. A link type is meant to be an "abstract" parent class for child classes or an esoteric link type only makes sense in a specific context.

Acceptance criteria

• A configuration flag is added to Link classes specifying if they should show by default in LinkField.
• Root Link class can not be enabled via this API no matter what.
  • All other built-in Link subclasses are enabled by default.
• Disabling a Link classes prevent it from showing as an option in a vanilla link field.
• Disabled Link class can be added back to specific LinkField instance via the setAllowedTypes method.
• The feature is documented in the developer docs.

PR

• https://github.com/silverstripe/silverstripe-linkfield/pull/219

[GuySartorelli]

Link is disable by default. All other built-in Link subclasses are enabled by default.

I think Link should just be explicitly excluded at all times. This makes it both easier to implement this config flag in a sensible way, and means people won't go "Ooh I can turn Link on" and have a non-funcitonal link implementation that doesn't actually link to anything.

[sabina-talipova]

PR's merged. Task is completed.