Improve labels for Move Attachments system utility

system_utils.php:

• changed description to clearly state that the action can move files
  both to database and to disk
• changed button label to remove "to disk"

move_attachments_page.php

• add tooltips for Storage column header and cells in "To Disk" and
  "To Database" columns
• remove obsolete <th> width attribute, improve table layout
• fix static analysis warnings

Fixes 0004993