DEV: Introduce declarative hide-application-footer helper (#23088)

Previously, we had a `showFooter` boolean on the application controller which would be set true/false in various routes by different routes/controllers. A global `routeWillChange` hook would set it `false` before every route transition, and the destination route/controller would have to set it `true` for the footer to show correctly.

This commit replaces that with a new 'declarative' system. Instead of having to set the value true/false manually, UIs which need the footer to be hidden can simply include the `{{hide-application-footer}}` helper in their template when needed. The helper/service will automatically keep track of all the current invocations of that helper, and only show the footer when there are 0 invocations.

This significantly simplifies things, and removes the need for many observers and controller injections, both of which are considered 'code smells' in modern Ember applications.

.template-lintrc.js

@@ -16,10 +16,11 @@ module.exports = {
         "directory-table-header-title",
         "loading-spinner",
         "directory-item-label",
+        "hide-application-footer",
       ],
     },
     "no-implicit-this": {
-      allow: ["loading-spinner"],
+      allow: ["loading-spinner", "hide-application-footer"],
     },
     "require-mandatory-role-attributes": false,
     "require-media-caption": false,