Email: Clear separation in courses-list

[ksteitz]

steps to reproduce:
Currently using the placeholder ##courses## results in all courses listet one after the other separated only by a space. That makes it very difficult to read and the different courses are hard to grasp.

improvement:
Insert line break after each course
Alternatively, if line brak isn´t possible for some reason: Other clear separation, e.g. half-quarter stroke (–) or quarter stroke (—) or a vertical bar/pipe ( | )

Looks like it can be addressed quite easily in the email step of the lib.php.