Change navbar position (sticky to static)

[mjyut]

Fix: #168

Issue #168 cannot be resolved without adding the z-index. However, pull request #169 has not been accepted.

So I thought of a non-hacky solution. Unfortunately, Bootstrap3 doesn't support sticky navbar. Static top is the most appropriate. Sticky position was cool, but it was also a disadvantage on narrow screens. I think it's an acceptable change.