* Add DeleteSidebarCategory method and channel category API tests
* Add test to verify channels move to default categories when custom category is deleted
* rename channel category tests and remove duplicate context
* TestNotifications fix
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Co-authored-by: Fume <contact@fumedev.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>