Change for default course activity URL when a custom course format is not supported.

[roperto]

This is after after merging #105 which introduces custom activity subpaths.

Some time ago a default activity path would be http://moodle.test/course/shortname/forum/123-myforum

After we implemented course format paths, they went inside a section, such as: http://moodle.test/course/shortname/topic-1/123-myforum and it would default to the previous path if a custom course format does not implement support for cleanurls (all core formats are supported).

After the PR #105 it is changing that default activity path to http://moodle.test/course/shortname/123-myforum dropping the modname from the address. This was a side effect from the code changes which I deliberately decided not to worry, as it affects only non-core course formats not supporting cleanurls but they are not broken, just receiving a different URL than what was planned before.

I am creating this issue only to register that change of behaviour and, if one day we decided to reimplement the old address format we can mention or work on this issue.