Update support for Silverstripe 5

[chrispenny]

SiteTreeLink no longer saves Page::$Title to Link::$Title

When we're using a SiteTreeLink, then it's quite common for us to want to inherit the Title of the linked Page. Current behaviour is that when you create your Link, the Page::$Title is saved into the Link::$Title. This means that if the Page::$Title changes later down the line, your Link will still have whatever the original Title was.

Updated logic:

• Added a getTitle() method to SiteTreeLink, which will first return Link::$Title, and fall back to Page::$Title (if possible).
• Removed the onBeforeWrite(), so we're no longer saving the Page::$Title in our Link model.
• Added TitleRelField to Link::jsonSerialize(), and updated the JS component to display this as a fall back if no Title was provided.

The outcome is...

In your Link preview, you can see the Title from TitleRelField:

But in the Edit form, the Title remains empty:

[chrispenny]

We've been running this branch on a Silverstripe 5 project for a while now, and haven't had any specific issues. I'll merge this and tag it as 3.0.0-beta1.