Compute hashes of executable files when migrating

The 'hash' column was added in commit bc1608f (Add hash to
executable files and immutable executables., 2021-03-30), but the
hashes were not computed for existing files, which broke migrations
from < 8.0.0 (judging fails with error message "Fetching executable
failed for compile script '<n>': Unexpected hash ...").

This commit fixes Version20210407120356.php so that it will compute
the hashes.

Note that this will not fix databases that have already been migrated
from 7.x using a DOMjudge version in the range 8.0.0 to 8.3.0. The
recommended workaround is to upgrade directly from 7.x to 8.3.1 (or
higher). Alternatively, there is a patch[1] that can be used to fix
such databases (it applies to 8.3.0, but the code can be adapted for
8.0).

[1] DOMjudge@f62d0ec

webapp/migrations/Version20210407120356.php

@@ -51,8 +51,8 @@ public function up(Schema $schema) : void
                         $executableBit = '0';
                     }
                     $this->connection->executeStatement(
-                        'INSERT INTO executable_file (`immutable_execid`, `filename`, `ranknumber`, `file_content`, `is_executable`) VALUES (?, ?, ?, ?, ?)',
-                        [$immutable_execid, $filename, $idx, $content, $executableBit]
+                        'INSERT INTO executable_file (`immutable_execid`, `filename`, `ranknumber`, `file_content`, `hash`, `is_executable`) VALUES (?, ?, ?, ?, ?)',
+                        [$immutable_execid, $filename, $idx, $content, $md5($content), $executableBit]
                     );
                 }