Merge PR https://github.com/mantisbt/mantisbt/pull/670

This PR fixes several issues with manage_config_workflow_page.php /
manage_config_workflow_set.php, and improves behavior and display.

This branch started off with @cproensa's PR [1], and evolved to include
the following changes

• Original commit (modified: whitespace fixes, reworded), fixes 0019970
• Fix a system warning introduced by the original commit
• Display color override for access level
• Proper fix for access level resets when updating workflow (0016771)
• Fix a regression introduced by b78f966e013d494ea27d92bba39d797a1266b7b7 (0020241)
• Do not display Delete button when there's nothing to delete (0006847)
• Adjust CSS/HTML and improve page layout

[1] https://github.com/mantisbt/mantisbt/pull/646