Usability bug fix

If the opacity slider was moved to the far right faster than the
animation showing the image, the user would never see the old file
because opacity got adjusted right away.

Now we first move the overlay slider to the right, so that something
is visible, and then quickly animate opacity to the current value.
Merge pull request #232 from tomaswolf/ticket-88-bugfix

Usability bug fix
Revise severity & priority permissions

Ticket reporters should be able to specify severity whereas only those with repository push access should be able to specify priority. This also fixes a bug related to non-admin users creating tickets.