Fix for btn-group-justify double borders

This is more of a proof of concept than a total fix. For this to fully work, the theme needs to be changed as well and rules need to be added for `open > .dropdown-toggle`. Those are both quite a bit of extra work and before I do that I would like to know if this solution is even appreciated. 

Furthermore any suggestions to make it more generic or less hacky are welcome. This was the best way I could think of.