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/Configuration/TCA/tx_vitec_domain_model_solution.php

@@ -24,7 +24,7 @@ return [
     ],
     'types'   => [
         '1' => [
-            'showitem' => 'title, slug, subtitle, teaser, description, image,
+            'showitem' => 'title, slug, detail_page, subtitle, teaser, description, image,
                 --div--;LLL:EXT:core/Resources/Private/Language/Form/locallang_tabs.xlf:categories, categories,
                 --div--;LLL:EXT:core/Resources/Private/Language/Form/locallang_tabs.xlf:language, sys_language_uid, l10n_parent, l10n_diffsource,
                 --div--;LLL:EXT:core/Resources/Private/Language/Form/locallang_tabs.xlf:access, hidden, starttime, endtime',
@@ -129,6 +129,17 @@ return [
                 'eval'              => 'uniqueInPid',
             ],
         ],
+        'detail_page'      => [
+            'exclude'     => true,
+            'label'       => 'Detail Page',
+            'description' => 'Page that presents this solution. Resolved to a URL in the JSON output; leave empty for no link.',
+            'config'      => [
+                'type'     => 'group',
+                'allowed'  => 'pages',
+                'size'     => 1,
+                'maxitems' => 1,
+            ],
+        ],
         'subtitle'         => [
             'exclude' => true,
             'label'   => 'Subtitle',