Don't allow users to create a wiki whose name starts with a number

It's currently possible for users to create wikis whose name starts with a number, which creates a lot of errors.

Steps to reproduce:

• Login as an admin
• Click on the "Wiki Manager" link on the right in the "Applications" panel
• Click on the "Create a new wiki" link
• Click on the "Switch to normal view mode" link
• Fill in the required fields (create_wiki.png)
• Click "Create"

You'll get an error (error.txt) after clicking on "Create". If you go in the Wiki Manager you'll see the wiki listed there (wiki_manager.png), but if you click on it you'll get another error (error2.txt).