Add moment.js for date related things (#50)

Fix saving

Use mixin everywhere

Format attachment dates

Add format date mixing

Use moment js on task list page

Use moment js on home page tasks

Add moment js

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/50

src/models/attachment.js

@@ -7,6 +7,7 @@ export default class AttachmentModel extends AbstractModel {
 		super(data)
 		this.created_by = new UserModel(this.created_by)
 		this.file = new FileModel(this.file)
+		this.created = new Date(this.created * 1000)
 	}
 
 	defaults() {