Two UI niceties for the slide well

*Adds padding to the bottom* so the "Add Slide" button is easily accessible on mouseover when the slide being appended to is at the very bottom of the list (removes the need to scroll further to access it & generally makes it easier/nicer to use).

*Adds a z-index* so slide content doesn't obstruct the slide well. Went with the same z-index that's already being used for the navbar (figured the navbar & slide well are equally important).
Increase z-index to coincide with slide well adding z-index

https://github.com/tantaman/Strut/pull/220 shows a need to add a z-index to the slide well, but this means the navbar-inner z-index needs to be increased so the dropdown isn't covered up.
Merge remote-tracking branch 'kzeni/patch-2' into kzeni-test
Merge remote-tracking branch 'kzeni/patch-3' into kzeni-test