Fix the "Message History" column from the Invitation app

The "Message History" column is displayed on 2 lines when you click on an Invitation. This makes the header row look taller than it should be.

To reproduce:

1. Set your SMTP server for the Invitation app: http://localhost:8080/xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=Invitation
2. Go to Invitation.WebHome and send an invitation, then accept it
3. Go to http://localhost:8080/xwiki/bin/view/Invitation/InvitationMemberActions?inspect=all
4. Click on the Accepted Invitation