Google OAuth2 authentication changes

[obaqueiro-primetrust]

I've got an implementation of Google OAuth2 signin (using the new Google Signin framework) available at https://github.com/prabushitha/gremlin-visualizer/compare/master...obaqueiro-primetrust:gremlin-visualizer:google_open_id . Ready to be merged once the Neptune change is made (the oauth2 changes are done on top of the others).

Additionally I have additional changes that will be adding in a separate PR with minor UI improvements (double click node to "expand" both in and out edges) and perform query search when pressing [Enter] in the query input text.

I plan on doing additional improvements to my fork:

• Implement some kind of "lazy" loading of nodes and edges. Because currently everything is done in 1 request to Gremlin server, we get a lot of Timeouts with queries that get lots of nodes. I can imagine that the hardcoded 100 limit was done as a workaround.
• Upgrade to newer version of React. There are several npm audit issues and deprecation notices.
• Implement some kind of UI "feedback" when a query is being performed (spinner in the query buttons, and spinner or something similar on top of the node that is being traversed)
• Better error communication in the frontend. Currently every error shows the "invalid query" text.
• Local storage of historic queries.

Long Term / Maybe

• Maybe the option of saving queries to a server so that they can be shared between people.