Fix OncoKBTable header/body misalignment when there is scrollbar

[calvinlu3]

Issue:
When the table body has a scrollbar, the header is not aligned with the body.

We try to fix this by adding the fixedHeight prop to OncoKBTable which applies some margin to the right (https://github.com/oncokb/oncokb-public/blob/master/src/main/webapp/app/index.scss#L284-L291). This is a quick fix, but introduces some other issues:

1. There is still some misalignment with 15px depending on the OS (mac vs windows) and by browser (different browsers have different scrollbar styles).
2. When we filter down the table to 1 entry (for example), the scrollbar disappears, but we still have the 15px margin right. As a result, the header/body is still misaligned.

Potential Solution(s):

1. We can add overflow-y: scroll to the body so that it always shows the scrollbar. (This will resolve the second issue). We can then try (https://davidwalsh.name/detect-scrollbar-width) to calculate the exact scrollbar width that we can use as margin. (This will resolve first issue)
2. Another option is to add a scrollbar to the header, but make the scrollbar background transparent to hide it.

Screenshot:
Misalignment when table body has scrollbar