Modify globus strategy to use single guest collection

[jdhayhurst]

Currently, we create a separate dir and guest collection for each submission. Instead, we can create one guest collection and each time create a subdir and add an ACL to that subdir. This cuts down on the overheads of managing so many collections.

[ljwh2]

Prioritise this in next sprint commencing 14th June

[jdhayhurst]

Top level function: sumstats_service.resources.globus.mkdir

Currently this function:

1. creates a new directory a submission
2. creates a "guest collection" to give the user access to that directory only.

Change this strategy to:
0. Setup a guest collection at the parent dir (not something we need to do once, not something that needs coding)

1. create a subdirectory inside the parent dir
2. add permissions to that subdir allowing the user access to the subdir only (can modify add_permissions_to_endpoint() to do this)
3. make sure that we can still get a link for the user to navigate to their subdir

[jdhayhurst]

ACLs are also limited to 1000 (per guest collection), so this solution doesn't change the fact that we can only have 1000 concurrent submissions open.

Recommend closing this as a non-issue and focusing on #252 instead