Default title for each link type

[maxime-rainville]

Story

As a CMS User I want the link to default to a sensible title based on it's type and the provided data so that I can have a good experience when editing links.

Acceptance criteria

• The "Default title" approach has been validated with UX team.
• Whenever the Link title field is left blank, the default title is substituted when rendering the link on the front end.
• The Linkfield's title field will stay blank when not provided.
• Feedback is provided to the user that they can leave LinkField title field blank.
• An approach has been implemented allowing developers to implement default title for each link type.
• The default title mechanic has been determined for each built-in link type and applied.
• Documentation has been provided to developers so they can implement a default title logic for their own link type.

Designs

Link field - Overview

Note

• Something like that has already been partially implemented. It might just be a question to validate that all they ACs have been met and/or refactoring things bit.

PR

• NEW Default link title for each link type #126

[brynwhyman]

More thinking required for this one from @silverstripeux. Some early thoughts discussed with @clarkepaul:

• Leave the title field blank
• Leave the title field blank, make it a required field
• Default to the URL of the link
• Custom logic for each link type to provide sensible defaults (like Page title, Email contact, Phone number, etc)

Noting that where data objects have been left without a title it's caused issues in the past where it comes to reporting on things like the Used On table. So I'd say that first option should be a no-go.

[GuySartorelli]

Merged. See #128 for a follow-up concern this has made me think of.