Fix header and list layout glitches in the WebUI

The new WebUI needs some fixes for several UI glitches:

• Add space around the OpenNMS Horizon Logo
• The Logo can't be read, remove the gradient image and replaced with grey color
• Removed text shadow effects in the start menu and changed from white font to black
• Added mouse hover in the menu to be more consistent with the hover effects in other places
• Removed the hover effect on panel titles, the old hover effect changed to an unreadable green.
• Flattened the indicator for severity issues to fit better the flat header
• Removed the gradient image for header and footer and replaced with header grey color
• Removed fixed font size to increase the space between lists.
• Removed the double line color in the cell which shows availability and amount of services
• Border around panel header set to the same color of the panel header

Changes are marked in the annotated screenshot.