Bound the menu fully within the container

[dandv]

The menu stays contained between the top and bottom boundaries of the container, but not between the left and right ones. This is far from a trivial problem - see HubSpot/drop#100.

[reedjones]

I had the same problem and the solution here worked: HubSpot/drop#16

I added the tether constraints in the selection-menu.js file (where it initializes tether (around line 242)
instance.tether = new Tether...

constraints: [ to: 'scrollParent', attachment: 'together', pin: true ]