Scope + menu dropdown to current organization

What does this MR do and why?

Related to #608081 and !253291 (merged).

When a user has adopted organizations, the + (Create new…) top-bar menu now scopes its create links to the relevant organization:

• General section — scopes to the request's organization (resolved from the /o/:organization_path segment or the group/project in the URL, including legacy global routes) and is titled In this organization instead of In GitLab. Links point to the organization-scoped pages (/o/:organization_path/...).
• Group section — scopes the New project/repository and New subgroup links to the group's own organization, so they create within it even when the group is reached via a legacy global route.

"Adopted organizations" is determined by Organizations::Release.enrolled?(current_user). The default organization is never URL-scoped (scoped_paths? is false), so its links stay global and the section keeps the In GitLab title.

The change is gated behind the organization_scoped_new_dropdown gitlab_com_derisk feature flag (see Feature flag section below).

Feature flag

This change is behind the organization_scoped_new_dropdown feature flag (type gitlab_com_derisk, default disabled) out of an abundance of caution. Should be low-risk change but we're putting Organizations::Release.enrolled? in a high-volume critical request path.

• Rollout issue: #629444

When "In this organization" appears

Assumes the organization_scoped_new_dropdown feature flag is enabled; when it is disabled the menu is always global (In GitLab).

Scenario	Section title	Link scope
SaaS (.com) — user enrolled (has a non-default active organization), viewing a scoped (non-default) organization or one of its groups/projects — including via a legacy global URL	In this organization	Org-scoped (/o/:organization_path/...)
SaaS (.com) — user enrolled, but the resolved organization is the default (unscoped) organization	In GitLab	Global
SaaS (.com) — user not enrolled (no non-default active organization)	In GitLab	Global
Self-managed / Dedicated — no enrollment flag passed, so Organizations::Release.enrolled? is always false	In GitLab	Global

Notes:

• "Enrolled" is Organizations::Release.enrolled?(current_user). On .com this is true when the user has a non-default active organization; on self-managed it requires an enrollment flag (none is passed here), so it is always false.
• The default organization is never URL-scoped (scoped_paths? is false), so even for enrolled users its links stay global.
• The group section scopes to the group's own organization; the general section scopes to the request's organization (resolved from the /o/:path segment or the group/project in the URL).

Screenshots or screen recordings

Notice the URL rendered in the bottom left corner of each screenshot, as well as which menu sections appear.

Note: Since these screenshots were posted, new organization and new snippet no longer appear in the "In this organization" menu.

Before	After

How to set up and validate locally

1. Go to /rails/features and enable the org_stage_experimental, org_stage_beta, and organization_scoped_new_dropdown feature flags.
2. Go to /o/new and create an organization.
3. Open the + menu — the general section should be titled In this organization and its links should be org-scoped.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.