Short labels for number of unread items

Code simplification
FreshRSS#473

FreshRSS@60f73f5

FreshRSS#100

There was an erroneous '; at the end of the commitNewEntries DELETE query, resulting in an exception during actualization with this output:

```
FreshRSS starting feeds actualization at 2021-01-11T14:06:09+00:00
FreshRSS actualize candrews...
PHP Fatal error:  Uncaught PDOException: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ''' at line 1 in /var/www/localhost/htdocs/FreshRSS/lib/Minz/Pdo.php:45
Stack trace:
#0 /var/www/localhost/htdocs/FreshRSS/lib/Minz/Pdo.php(45): PDO->exec()
#1 /var/www/localhost/htdocs/FreshRSS/app/Models/EntryDAO.php(167): Minz_Pdo->exec()
#2 /var/www/localhost/htdocs/FreshRSS/app/Controllers/feedController.php(505): FreshRSS_EntryDAO->commitNewEntries()
#3 /var/www/localhost/htdocs/FreshRSS/app/Controllers/feedController.php(548): FreshRSS_feed_Controller::actualizeFeed()
#4 /var/www/localhost/htdocs/FreshRSS/lib/Minz/Dispatcher.php(119): FreshRSS_feed_Controller->actualizeAction()
#5 /var/www/localhost/htdocs/FreshRSS/lib/Minz/Dispatcher.php(47): Minz_Dispatcher->launchAction()
#6 /var/www/localhost/htdocs/FreshRSS/lib/Minz/FrontController.php(81): Minz_Dispatcher->run()
#7 /var/www/localhost/htdocs/FreshRSS/app/actualize_script.php(75): Minz_FrontController->run()
#8 {main}
  thrown in /var/www/localhost/htdocs/FreshRSS/lib/Minz/Pdo.php on line 45
Results:
```

Fix:
> Deprecated:  DOMElement::setAttributeNS(): Passing null to parameter #3 ($value) of type string is deprecated in /var/www/FreshRSS/lib/marienfressinaud/lib_opml/src/LibOpml/LibOpml.php on line 680

Fix:
> Deprecated:  DOMElement::setAttributeNS(): Passing null to parameter #3 ($value) of type string is deprecated in /var/www/FreshRSS/lib/marienfressinaud/lib_opml/src/LibOpml/LibOpml.php on line 680

Not really used, and causing bug in FreshRSS#5942

```
Fatal error: Uncaught Error: Typed property FreshRSS_View::$default_category must not be accessed before initialization in /var/www/FreshRSS/app/views/helpers/category/update.phtml:218 Stack trace: #0 /var/www/FreshRSS/lib/Minz/View.php(93): include() #1 /var/www/FreshRSS/lib/Minz/View.php(143): Minz_View->includeFile() #2 /var/www/FreshRSS/app/views/category/update.phtml(9): Minz_View->renderHelper() #3 /var/www/FreshRSS/lib/Minz/View.php(93): include('...') #4 /var/www/FreshRSS/lib/Minz/View.php(115): Minz_View->includeFile() #5 /var/www/FreshRSS/lib/Minz/View.php(75): Minz_View->render() #6 /var/www/FreshRSS/lib/Minz/Dispatcher.php(59): Minz_View->build() #7 /var/www/FreshRSS/lib/Minz/FrontController.php(61): Minz_Dispatcher->run() #8 /var/www/FreshRSS/p/i/index.php(59): Minz_FrontController->run() #9 {main} thrown in /var/www/FreshRSS/app/views/helpers/category/update.phtml on line 218
```

Not really used, and causing bug in FreshRSS#5942

```
Fatal error: Uncaught Error: Typed property FreshRSS_View::$default_category must not be accessed before initialization in /var/www/FreshRSS/app/views/helpers/category/update.phtml:218 Stack trace: #0 /var/www/FreshRSS/lib/Minz/View.php(93): include() #1 /var/www/FreshRSS/lib/Minz/View.php(143): Minz_View->includeFile() #2 /var/www/FreshRSS/app/views/category/update.phtml(9): Minz_View->renderHelper() #3 /var/www/FreshRSS/lib/Minz/View.php(93): include('...') #4 /var/www/FreshRSS/lib/Minz/View.php(115): Minz_View->includeFile() #5 /var/www/FreshRSS/lib/Minz/View.php(75): Minz_View->render() #6 /var/www/FreshRSS/lib/Minz/Dispatcher.php(59): Minz_View->build() #7 /var/www/FreshRSS/lib/Minz/FrontController.php(61): Minz_Dispatcher->run() #8 /var/www/FreshRSS/p/i/index.php(59): Minz_FrontController->run() #9 {main} thrown in /var/www/FreshRSS/app/views/helpers/category/update.phtml on line 218
```