Make problem.externalid a unique key.

This was only meant to be unique per contest, but this caused
subtle issues with the POST /api/problems endpoint. In that case,
there might be multiple problems with the given externalid and
if this is used as external problem ID, then that would fail.

Note that removing the UniqueEntity message just means that a
default message about duplicate keys will be shown e.g. in forms
when you try to add one. This is consistent with the settings we
have for other entities.

In more detail:

If there is already a problem with externalid `foo`, but not
associated to any contest, and you try to add a contest that also
contains a problem with externalid `foo`, then first that problem
gets created (which works), but after that when data is loaded at
https://github.com/DOMjudge/domjudge/blob/8231f1291effadb328161462dd86a9f5223fc025/webapp/src/Service/ImportProblemService.php#L856-L859
if `externalIdFieldForEntity` is `externalid`, `getOneOrNullResult`
throws an exception.
I tried fixing this case by adding a check on the same contest,
but this code is also called from POST /api/problems without the
context of a contest. In that case it becomes unclear how to
handle this, especially if there are already multiple problems
with that externalid: should it create a new one or edit (which?)
one of the existing ones?
To avoid this complication I decided it is easier to just have
a unique external ID across problems globally.

(cherry picked from commit 6ace309)

Author: eldering

webapp/migrations/Version20230508153514.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20230508153514 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Make problem.externalid a unique index.';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('DROP INDEX externalid ON problem');
+        $this->addSql('CREATE UNIQUE INDEX externalid ON problem (externalid(190))');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('DROP INDEX externalid ON problem');
+        $this->addSql('CREATE INDEX externalid ON problem (externalid(190))');
+    }
+
+    public function isTransactional(): bool
+    {
+        return false;
+    }
+}

webapp/src/Entity/Problem.php

@@ -20,6 +20,12 @@
  * @ORM\Entity()
  * @ORM\Table(
  *     name="problem",
+ *     uniqueConstraints={
+ *          @ORM\UniqueConstraint(
+ *              name="externalid",
+ *              columns={"externalid"}
+ *          )
+ *     },
  *     options={"collation"="utf8mb4_unicode_ci", "charset"="utf8mb4","comment"="Problems the teams can submit solutions for"},
  *     indexes={
  *         @ORM\Index(name="externalid", columns={"externalid"}, options={"lengths": {190}}),