Fix nav bar for mobile view

[benheng]

No description provided.

[benheng]

Hidden navigation with hamburger button:

Hamburger button clicked opens a vertical nav list:

[andyhuang91]

LGTM. Since we are using react-bootstrap, you could also simplify the rest of the navigation file by using those components.

[kwm4385]

@benheng Since the release process for Singularity is a bit different, we'll need to merge and deploy this through our three environment branches in sequence. These are hs_staging, hs_qa and hs_stable. After you merge your feature branch into one of these, add the corresponding label on the PR. Let me or Andy know if you have any questions!