Add getPlagiarismRecordsCount function, and show open case count

[theresnotime]

Adds a function named getPlagiarismRecordsCount, which accepts the same filtering options as getPlagiarismRecords, has a higher limit (200) and caches the count result for 15 minutes.

This is called in AppController as such:

$open_records_count = $copyPatrolRepo->getPlagiarismRecordsCount(
	array_merge(
		$options,
		[
			'filter' => CopyPatrolRepository::FILTER_OPEN
		]
	)
);
// If there are 200 (or more) open records, we just show "200+".
if ( $open_records_count >= 200 ) {
	$ret['open_records_count'] = "200+";
} else {
	$ret['open_records_count'] = $open_records_count;
}

Requesting more than 200 records to count appears to be detrimental to performance, so currently if the result is 200 (or more), the string 200+ is displayed.

The count is wrapped in <span id="open_records_count" data-count="{{ open_records_count }}">{{ open_records_count }}</span> to make it marginally easier to parse the page with a bot/etc (as it was mentioned that an on-wiki rough count would also be beneficial)

Bug: T375528