Export button click will now be disabled until a report is selected

Prevented a user click on the Export button by graying and disabling it.
The button will be enabled only when a report is selected from the
Available Reports list.

https://bugzilla.redhat.com/show_bug.cgi?id=1110244
https://bugzilla.redhat.com/show_bug.cgi?id=1101222
Merge pull request #88 from AparnaKarve/bz1110244_prevent_export_buttonclick

Export button click will now be disabled until a report is selected