Remove input-prepend and input-append for singular .input-group. The classes have changed for the elements within, and require a bit more markup to use buttons.
Use .input-group as the parent class around the input and addon.
For text based prepends/appends, use .input-group-addon instead of .addon.
For button prepends/appends, use .input-group-btn and place your .btn within that element.

Fix form controls' sizes.

See https://github.com/twbs/bootstrap/pull/6342