Data layer restructuring (#997)

* Move query builders to sqlite package
* Add transaction system
* Wrap model resolvers in transaction
* Add error return value for StringSliceToIntSlice
* Update/refactor mutation resolvers
* Convert query builders
* Remove unused join types
* Add stash id unit tests
* Use WAL journal mode

pkg/models/model_gallery.go

@@ -42,3 +42,13 @@ type GalleryPartial struct {
 }
 
 const DefaultGthumbWidth int = 640
+
+type Galleries []*Gallery
+
+func (g *Galleries) Append(o interface{}) {
+	*g = append(*g, o.(*Gallery))
+}
+
+func (g *Galleries) New() interface{} {
+	return &Gallery{}
+}