Minor styling tweaks for external resource button in LTI component.

LMS-11302

Minor styling tweaks for external resource button in LTI component.

Reset to previous commit: https://github.com/edx/edx-platform/commit/06a85e6ef53264efd2379447f2a3b147150ed568
@adampalay @talbs we can align the button to vertically middle by doing this ```$baseline/2```