This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.
These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.
This adds support for only allowing normal users with account age
equal or greater than a "waiting period" threshold to create streams;
this is useful for open organizations that want new members to
understand the community before creating streams.
If create_stream_by_admins_only setting is set to True, only admin users
were able to create streams. Now normal users with account age greater
or equal than waiting period threshold can also create streams.
Account age is defined as number of days passed since the user had
created his account.
Fixes: #2308.
Tweaked by tabbott to clean up the actual can_create_streams logic and
the tests.