Remove all remaining occurences of fakeApp (#17661)

* Remove all remaining occurences of fakeApp

Migrated any remaining methods under server.

And for everything else, renamed fakeApp to app.
The word fakeApp is confusing and we should just call
it for what it is - an app.

https://focalboard-community.octo.mattermost.com/workspace/zyoahc9uapdn3xdptac6jb69ic?id=285b80a3-257d-41f6-8cf4-ed80ca9d92e5&v=495cdb4d-c13a-4992-8eb9-80cfee2819a4&c=639a0bc1-4401-43d5-81ec-0dd54e796d9a

```release-note
NONE
```

* fix tests

app/plugin_requests_test.go

@@ -40,7 +40,7 @@ func TestServePluginPublicRequest(t *testing.T) {
 		require.NoError(t, err)
 
 		rr := httptest.NewRecorder()
-		handler := http.HandlerFunc(app.ServePluginPublicRequest)
+		handler := http.HandlerFunc(srv.ServePluginPublicRequest)
 		handler.ServeHTTP(rr, req)
 
 		assert.Equal(t, http.StatusNotFound, rr.Code)