Improve the "Last Author" field to display the author's avatar on AWM livetables

Steps to reproduce:

1. Go to "More Applications" from "Application" panel
2. Click on "Create your own"
3. Start creating an app
4. Add an "User" field
5. Click on "Next" button
6. You see the "Step 3"
7. Add your new added field on the livetable by adding it from the list and do not delete any existing default fields ("Page name", "Update date", "Last Author", "Date")
8. After you save the app, go to the app homepage
9. Add a new entry
10. The user from "User" field has an icon (avatar) and the "Last Author" (even there is an user too) has no icon

Objective

Use the proper user displayer for the Author livetable column.

Requirements

• Knowledge of HTML, XML, basic programming, Git
• You will encounter concepts like Data Model, 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

Familiarize yourself with XWiki by downloading and installing the latest version.

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

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.
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.