Replace paste0 by make_canvas_url

[vanatteveldt]

I noticed the group functions no longer worked (at least get_groups_context and get_group_categories), giving a 404 because of a wrongly constructed URL:

[1] "https://canvas.vu.nl/api/v1courses/72446/groups"

(note the missing slash after /api/v1)

I figured that that the rcanvas:::canvas_url() was probably changed to no longer have a trailing slash. In any case, other methods seemed to use make_canvas_url instead of manually pasting the components, so I changed all functions in that source file to use this function instead.

Please note that I've tested this for the get_* functions, but as I don't have access to a test environment I did not test the membership changing functions, but the code changes look trivial enough to me :D