change subscriptions admin

[Ruben Van Wassenhove]

since there are multiple courses assigned to a subscription, you can not just display 1 course, so:

• add a column 'number of subscriptions' with link
• if you click the link, go to a page where you see a list of courses
• change the filter code, so that it works for subscriptions with multiple courses
• change the export to excel code (see issue #42)

[Ruben Van Wassenhove]

just added an s for m2m filters to fix #44

→ <<cset 60fe5c690e0c>>