Fix up the Podcast Cover Setter.

This involved removing a function that was defined in both common.php
and setCover.php.

While I was at it, I also added a newline into common.php after the
<?, for consistency with my other code.
Fix User type test in MyRadioEmail.

The previous code tested against a class that no longer exists, and
also used 'is_a', which seems to be discouraged in favour of
'instanceof'.

This commit changes the class to the correct class, MyRadio_User,
and also changes from is_a to instanceof.  The exception has also
been slightly reworded.
Merge branch 'develop' of https://www.github.com/UniversityRadioYork/MyRadio into develop