Fix 'max key length is 767 bytes' errors (Closes: #886756)

Allow index keys up to 3072 bytes. Many apps expect VARCAHR(255) to work.
Since we introduced UTF-8, that has not been the case. This can be fixed
via option 'innodb_default_row_format' that was backported to 10.0.x
series recently.

The option 'innodb_file_per_table' is 'on' by defautl since MariaDB 5.5,
so there is no need to define it.

See also
https://jira.mariadb.org/browse/MDEV-14904
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=848603
https://mariadb.com/kb/en/library/xtradbinnodb-server-system-variables/#innodb_file_format

debian/additions/mariadb.conf.d/50-server.cnf

@@ -78,12 +78,6 @@ max_binlog_size   = 100M
 #binlog_do_db		= include_database_name
 #binlog_ignore_db	= exclude_database_name
 
-#
-# * InnoDB
-#
-# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
-# Read the manual for more InnoDB related options. There are many!
-
 #
 # * Security Features
 #
@@ -111,6 +105,18 @@ max_binlog_size   = 100M
 character-set-server  = utf8mb4
 collation-server      = utf8mb4_general_ci
 
+#
+# * InnoDB
+#
+# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
+# Read the manual for more InnoDB related options. There are many!
+#
+# Allow index key prefixes up to 3072 bytes. If not set, the limit is 767 bytes.
+# This is important as utf8mb4 is used. See Debian bug #886756.
+innodb_file_format        = barracuda
+innodb_large_prefix       = on
+innodb_default_row_format = dynamic
+
 #
 # * Unix socket authentication plugin is built-in since 10.0.22-6
 #