SQL concat() function is not 100% cross-db

While nowadays all the main databases have a CONCAT() function (some years ago that was not the case), they don't provide a full cross-db experience.

• Some are not able to concatenate non-char types.
• Others have a non-standard (SQL92 from memory) behaviour with NULLs.

Because of that, the sql_concat() helper method was created. It provides custom code to make all databases to behave consistently.

Right now there are a few (3 I've been able to find, though the search was not super-intensive) cases in core using the SQL native function:

$ ag -i ' concat\(' --php | ag 'SELECT|FROM|AND|OR|WHERE|ORDER'

lib/classes/task/manager.php:989:                  FROM (SELECT concat('s', ts.id) as uniqueid,

lib/classes/task/manager.php:1000:                        SELECT concat('a', ta.id) as uniqueid,

h5p/tests/h5p_file_storage_test.php:821:        $sql = "SELECT concat(filepath, filename)

This issues is about to move them to use the helper function and verify that everything continues working ok (it should).

Ciao