Add route + feature flag for project storage UI

What does this MR do?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/334882

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/334883

This is the initial work of Project Storage UI, which adds the feature flag and the route for usage_quotas at project level

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Enable project_storage_ui feature flag. Go to rails console and execute the following:

   Feature.enable(:project_storage_ui)

2. Visit /<namespace>/<project>/-/usage_quotas on a project you have maintainer permission
3. You should see a page like the screenshot above

• If you don't have maintainer permission on a project, you should see a 404 page
• If you have project_storage_ui feature flag disabled, you should see a 404 page

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.