Add feature flag for making organization a required argument

What does this MR do and why?

As part of #443494 (closed), we are adding support for passing in an Organization when creating a Personal Namespace. We will require an Organization to be present on newly created Namespaces.

In a serie of MR's, we modified all the code paths that are using User.assign_personal_namespace method and we ensured an Organization is now passed to the method. We are now ready to make Organization a required argument. In order to reduce risk, changing to required argument will be behind a derisk Feature Flag.

This allows us to quickly make organization an optional argument, in case we missed some code path.

This MR adds a feature flag personal_namespace_require_org:

• If enabled, User.assign_personal_namespace requires an Organization argument
• If disabled, User.assign_personal_namespace, the argument is optional

Behavior when the flag is enabled:

# Allowed
User.new.assign_personal_namespace(Organizations::Organization.find(1)) # Pass organization
User.new.assign_personal_namespace(nil) # Required argument but it is still ok to be `nil`

# Not allowed: Argument error
User.new.assign_personal_namespace

Behavior when the flag is disabled:

# Allowed
User.new.assign_personal_namespace(Organizations::Organization.find(1)) # Pass organization
User.new.assign_personal_namespace(nil)
User.new.assign_personal_namespace

MR acceptance checklist

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How to set up and validate locally

Using rails console:

[1] pry(main)> Feature.enabled? :personal_namespace_require_org
=> false
[2] pry(main)> User.new.assign_personal_namespace # No error, returns new (unsaved) Namespace
=> #<Namespaces::UserNamespace id: @>
[3] pry(main)> Feature.enable :personal_namespace_require_org
=> true
[4] User.new.assign_personal_namespace
ArgumentError: Organzation is missing
from /path/app/models/user.rb:1688:in `assign_personal_namespace'