[CloudSAASOpenbeta] - Change user menu organization navigation for open beta

Created by: pietrorosa77

This PR it's the first of a series of pr for the Open beta release. We are changing the user navigation menu and account menu removing the Organization list and adding a "your organizations" entry that will redirect the user to a page with the list of organizations the user belongs to

https://sourcegraph.atlassian.net/browse/CLOUD-296 https://sourcegraph.atlassian.net/browse/CLOUD-297

Test plan

The code is entirely behind a Feature flag and it has no impact on the existing