PLT-4938 Add app package and move logic over from api package (#4931)

* Add app package and move logic over from api package

* Change app package functions to return errors

* Move non-api tests into app package

* Fix merge

api/context_test.go

@@ -4,32 +4,9 @@
 package api
 
 import (
-	"github.com/mattermost/platform/model"
 	"testing"
 )
 
-func TestCache(t *testing.T) {
-	session := &model.Session{
-		Id:     model.NewId(),
-		Token:  model.NewId(),
-		UserId: model.NewId(),
-	}
-
-	sessionCache.AddWithExpiresInSecs(session.Token, session, 5*60)
-
-	keys := sessionCache.Keys()
-	if len(keys) <= 0 {
-		t.Fatal("should have items")
-	}
-
-	RemoveAllSessionsForUserId(session.UserId)
-
-	rkeys := sessionCache.Keys()
-	if len(rkeys) != len(keys)-1 {
-		t.Fatal("should have one less")
-	}
-}
-
 func TestSiteURL(t *testing.T) {
 	c := &Context{}