Add View for Saas User Cap Notifications

What does this MR do and why?

Adds the view for the namespace user cap limit alert described in #332595 (closed).

The view is not displayed anywhere yet. A future MR will display the banner as appropriate.

This is following a pattern set by the namespace storage limit alert.

This follows the designs for the banner: #321934[limit_reached.png]

Issue: #332595 (closed)

Screenshots

On the Group overview page:

On a project overview page, with other alerts present:

How to set up and validate locally

You can make the view appear on any group or project page with the following patch:

diff --git a/app/views/layouts/_page.html.haml b/app/views/layouts/_page.html.haml
index ec2904245d3..39049afc2a6 100644
--- a/app/views/layouts/_page.html.haml
+++ b/app/views/layouts/_page.html.haml
@@ -17,6 +17,7 @@
       = render_two_factor_auth_recovery_settings_check
       = render_if_exists "layouts/header/ee_subscribable_banner"
       = render_if_exists "shared/namespace_storage_limit_alert"
+      = render_if_exists "shared/namespace_user_cap_reached_alert"
       = render_if_exists "shared/new_user_signups_cap_reached_alert"
       = yield :page_level_alert
       = yield :customize_homepage_banner

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.