Block config setting to allow students to send mails without effect.

[krostas1983]

If in the site administration the setting block_quickmail_allowstudents is set to "Yes" or "No" as default value, leaving it at "Yes" or switching to "Yes" will have no effect - students are not allowed to send mails regardless of the setting. This is because in line 45 of classes/plugin.php, the default value of $includestudentaccess=true is being overwritten with false.

In #98, this is fixed and a check is being added whether the admin setting block_quickmail_allowstudents is set no "Never" (in which case false is correct).