Solution detail page and a single LinkResolver

Closes two annex items that turned out to be the same defect seen from two
sides: markets could link to a detail page, solutions could not, and the code
that turns a page uid into a URL existed four times over.

detail_page for Solution (B-11)
- ext_tables.sql, TCA (group on pages, maxitems 1, in showitem behind slug)
  and Domain\Model\Solution mirror the Market field exactly
- SolutionShowJsonRenderer emits detailUrl
- ModelcardJsonRenderer serialized no detailUrl at all: its shared
  market|solution branch never carried the field. Market modelcards therefore
  gain a link here too, not just solutions.

Service\LinkResolver (B-12)
- one implementation replaces four private copies (Market list/show, Usecase
  list/show). The copies had drifted: Market returned null on failure, Usecase
  an empty string. The contract is now explicit -- null means "no link".
- Usecase keeps a thin resolvePageUrl() wrapper appending ?? '', because it
  concatenates paths and a null would tear the strings apart.
- LocationsJsonRenderer and NewsJsonRenderer stay out on purpose: they resolve
  a full parameter construct resp. slug paths plus canonical, which is not
  page-uid-to-URL and does not fit the same contract.

Requires on the server, the column exists in code only:
    vendor/bin/typo3 database:updateschema "*.add,*.change"
    vendor/bin/typo3 cache:flush

packages/vitec/Classes/Domain/Model/Solution.php

@@ -18,6 +18,13 @@ class Solution extends AbstractEntity
      */
     protected $slug = '';
 
+    /**
+     * Page uid presenting this solution (TCA type "group", allowed: pages).
+     *
+     * @var int
+     */
+    protected $detailPage = 0;
+
     /**
      * @var string
      */
@@ -86,6 +93,22 @@ class Solution extends AbstractEntity
         $this->slug = $slug;
     }
 
+    /**
+     * @return int
+     */
+    public function getDetailPage(): int
+    {
+        return $this->detailPage;
+    }
+
+    /**
+     * @param int $detailPage
+     */
+    public function setDetailPage(int $detailPage): void
+    {
+        $this->detailPage = $detailPage;
+    }
+
     /**
      * @return string
      */