The current logic for showing discussion providers makes it hard to switch
from the legacy to the new provider. This commit changes the conditions in
which different providers are shown, and which provider is used as default.
Before this commit, the new provider would be hidden if the legacy provider
was in use and vice-versa. So both would only be shown if neither legacy
nor the new provider were in use (i.e. an LTI provider was in use).
Now, all providers are always displayed to global staff. If the waffle
flag for the new provider is set
(`discussions.enable_new_structure_discussions`), then new provider is
always displayed, and the legacy provider is hidden unless it's currently
in use.
If flag is not set, then the new provider is always hidden unless it is
used by a course.
Finally, the default provider now depends on the flag above. If it is
set globally, then the default provider is the new provider, otherwise
the legacy provider remains the default provider.
c2326f0399