Use .xform style standard in Panels

Issue suggested by vmassol.

Objective

Make sure the Panels sheet is using the .xform standard, see http://platform.xwiki.org/xwiki/bin/view/DevGuide/VerticalForms, in order for the inputs to be the same size and consistent with the other forms through XWiki.

Requirements

• Knowledge of HTML, CSS, Bootstrap, XML, Git
• You will encounter concepts like Data Model, Skins Extensions, Templating engines, Apache Velocity, modular components, etc. but you don't need to be familiar with them in advance

Expected Outcome

• Pull Request on the https://github.com/xwiki/xwiki-platform repository with the solution
• The summary of the Pull Request must have a first line matching the JIRA issue following by a colon and then the title of this issue
• Following best practices defined at http://dev.xwiki.org/xwiki/bin/view/Main/WebHome

Prerequisites

Before starting your task familiarize yourself a bit with XWiki. You can see it in action in the Playground, so you should create an account there and test it a bit.
Your task might involve actions or applications that are not available on the Playground, so make sure you download and install the latest version of XWiki.

Try to reproduce the issue described by the task. You should use your browser's Developer Tools to identify and test your solution.

Once you have reproduced the issue presented in the task, it's the time to identify the location where it needs to be fixed in the code.
The sources are available at https://github.com/xwiki/xwiki-platform . You should be familiar with using Git locally or you might be able to commit your changes from the GitHub interface.

The task will give you hints about the component where the code with the problem is located. Identify the file, make the changes and submit a Pull Request for mentors to approve and close the task. Don't forget to add screenshots with before and after your changes.

For questions you can always use the Forum or the IRC channel, see http://dev.xwiki.org/xwiki/bin/view/Community/Discuss