Send mails with error information to site administrator

Fixes #136

.env.prod

@@ -0,0 +1,2 @@
+MAILER_DSN="native://default"
+ERROR_LOG_MAIL="error@radiolista.pl"

config/packages/prod/monolog.yaml

@@ -3,13 +3,30 @@ monolog:
         main:
             type: fingers_crossed
             action_level: error
-            handler: nested
+            handler: group
             excluded_http_codes: [404, 405]
             buffer_size: 50 # How many messages should be saved? Prevent memory leaks
-        nested:
+        group:
+            type: group
+            members: [file, mail_deduplicate]
+
+        file:
             type: stream
             path: "%kernel.logs_dir%/%kernel.environment%.log"
             level: debug
+
+        mail_deduplicate:
+            type: deduplication
+            handler: mail_send
+        mail_send:
+            type: symfony_mailer
+            from_email: '%env(ERROR_LOG_MAIL)%'
+            to_email: '%env(ERROR_LOG_MAIL)%'
+            subject: '%%message%%'
+            level: debug
+            formatter: monolog.formatter.html
+            content_type: text/html
+
         console:
             type: console
             process_psr_3_messages: false