BUG Tweak URL generation so blocks can be edited when added to an Area on a regular DO

[maxime-rainville]

This is a WIP. Not ready for peer review.

Parent issue

• Fixes Elemental Area can't easily be added to a non-Page DataObject and edited in a ModelAdmin #871

[maxime-rainville]

A dev would need to add this code snippet to their DO for this PR to work.

    /**
     * Sanitise a model class' name for inclusion in a link
     *
     * @param string $class
     * @return string
     */
    protected function sanitiseClassName($class)
    {
        return str_replace('\\', '-', $class);
    }

    public function CMSEditLink()
    {
        return Controller::join_links(
            TestModelAdmin::singleton()->Link($this->sanitiseClassName(self::class)),
            'EditForm/field',
            $this->sanitiseClassName(self::class),
            'item',
            $this->ID,
            'edit'
        );
    }

    public function Link() {
        return 'frontend-link';
    }