GUI-881: Add "Select group…" text to group drop-down on user detail page.  Disable button if no groups are available to add.  Minor CSS fix for disabled button hover bg color.
GUI-881: Ensure groups drop-down is alphabetically sorted and "Select group…" is displayed after add/remove operations.
Merge pull request #645 from eucalyptus/GUI-881

GUI-881: Add "Select group…" text to group drop-down on user detail page