[#809] fix: SQL input field width to improve UX for larger queries

[keevan]

Resolves #809

Before:

After:

[keevan]

I tried using a class (w-100) instead of adding to the styles, but I don't think I'm applying it properly.

e.g. these didn't work:

        $mform->addElement('textarea', 'config_sql', get_string('reader_sql:sql', 'tool_dataflows'),
            ['class' => 'w-100', 'cols' => 50, 'rows' => 7, 'style' => 'font: 87.5% monospace;']);

        $mform->addElement('textarea', 'config_sql', get_string('reader_sql:sql', 'tool_dataflows'),
            ['class' => 'w-100', 'rows' => 7, 'style' => 'font: 87.5% monospace;']);

[Peterburnett]

This makes me wonder whether this should be an abstract class instead. This is more an idle thought, no changes required.