Api eventContext

[will-moore]

What this PR does

Adds support for /api/v0/eventcontext/ which returns the current eventContext.
Needed for clients to retrieve the current context some time after logging in.

Needs discussion:

We need to restrict public_user from accessing this since they will be able to access their sessionId and join session as a fully logged-in public user.

Testing this PR

1. Go to /api/v0/eventcontext/ after logging in.
2. Should see event contest JSON data.

TODO:

• Add test

[will-moore]

Tests failing with

'url:eventcontext': build_url(request, 'api_eventcontext'. v),\nAttributeError: 'str' object has no attribute 'v'\n", "message": "'str' object has no attribute 'v'"}

e.g. https://ci.openmicroscopy.org/view/Failing/job/OMERO-DEV-merge-integration-Python27/646/testReport/OmeroWeb.test.integration.test_api_containers/TestContainers/test_container_crud_Project_/

[will-moore]

As for #5315, this is not needed for 5.4.0 and still requires tests etc. Closing now to focus on release...