Make update site description read-only

Edits here weren't actually saved anyway. The current mechanism for
updating the description is to edit https://imagej.github.io/list-of-update-sites/

Closes imagej/imagej-updater#119

Author: hinerm

src/main/java/net/imagej/ui/swing/updater/SitesDialog.java

@@ -130,6 +130,8 @@ public void valueChanged(final ListSelectionEvent e) {
 
 			@Override
 			public boolean isCellEditable(final int row, final int column) {
+				// Make Description column read-only
+				if ("Description".equals(getColumnName(column))) return false;
 				return column >= 0 && column < getColumnCount() && row >= 0 && row < getRowCount();
 			}