Replace placeholder with NamespaceStorageApp

What does this MR do and why?

This is the last of a series of MRs that aim to migrate the Namespace Storage Vue App to CE so that this view is available for all GitLab Editions and Distributions. Related to: Move storage app to CE (#371824 - closed)

Now that we've migrated the app to the CE directory, we can initiate the app in the CE view so that it's rendered for CE users.

MR Cadence

Description	MR
Introduce NamespaceStorageApp in core	!138689 (merged)
Create StorageUsageStatistics in CE	!139111 (merged)
Move DependencyProxyUsage to CE	!139248 (merged)
Move ContainerRegistryUsage to CE	!139252 (merged)
Move SearchAndSortBar to CE	!139434 (merged)
Move ProjectList to CE	!139449 (merged)
Finish moving NamespaceStorageApp to CE	!139568 (merged)
Replace placeholder with NamespaceStorageApp	👈 you're here

Screenshots or screen recordings

Feature Flag Off (No changes)

Before	After

Feature Flag On

Before	After

How to set up and validate locally

We need to validate this in 4 scenarios:

1. GitLab EE Unlicensed: this is probably how your GDK is right now. If you have a license in your GDK, remove it in https://gdk.test:3443/admin/subscription
2. GitLab EE: by having a test license in your GDK.
3. GitLab SaaS: setup a license like the above and simulate SaaS
4. GitLab CE: by executing
   1. export FOSS_ONLY=1
   2. gdk restart
   3. Tanuki gdk website icon (the one in the browser tab) should be blue, then you're in CE

Now go to any Group Usage Quotas: https://gdk.test:3443/groups/flightjs/-/usage_quotas and check if your local setup matches the expected behavior.

• To enable the FF: Feature.enable(:usage_quotas_for_all_editions)
• To disable the FF: Feature.disable(:usage_quotas_for_all_editions)

Expected behavior:

• Expected behavior with Feature Flag on: for all the scenarios, the page loads only the storage tab, and shows the data normally, hiding SaaS UI when not in SaaS.

GitLab Distribution	Expected behavior FF off
GitLab EE Unlicensed	Renders a 404 page
GitLab EE	Loads the page normally, hiding SaaS UI (e.g. the right hand side card in Namespace overview)
GitLab SaaS	Page loads normally, like it is on master/GitLab.com
GitLab CE	Renders a 404 page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.