Fix English labels

Some english labels need to be fixed. I put my suggestions below:

• user.center.nuxeoDrive.platform.osx=Mac OS X
• documentCheckedOut= Document checked out
• documentImported= Document imported
• RICH_FILE_UPLOAD_UPLOAD_LABEL=Resuming
• label.setup.table.title.httpProxy=HTTP Proxy
• adminsubtab.usersActivity.userSessions.total=Total number of active HTTP sessions:
• adminsubtab.usersActivity.requests.total=Total number of HTTP requests:
• adminsubtab.sysinfo.repostat.activeconnection=Number of active connections:
• label.maintenancemode.adminLoginOnly=Only administrators can access the server
• label.maintenancemode.message=Message from administrators
• message.error.pdf.openOffice=Nuxeo Platform does not succeed to establish a connection to the OpenOffice.org/LibreOffice process.
  Note about this label: we should add some details about possible reasons and how to solve the issue (LIbreOffice is not installed or configuration is incorrect, first connection failed, connection was lost).
• action.acl.audit.export.tooltip=Start a permission audit in a background work. Once started, its progress can be monitored from the Admin tab. The result is sent via email.
• label.setup.nuxeo.ldap.default.membersGroup=Default group