Basic unit testing setup using jest

[drenther]

Subject

Adding basic unit testing structure

Changes

• added eslint-plugin-jest
• added eslint env jest: true
• added jest as devDep to client using create-react-app jest (changes reverted)
• added jest and supertest as devDep to server
• added dummy test under _tests_ in server
• moved mongoose setup to server.js to make app more testable

[skyshader]

Let's add the test script into pre-commit hook along with our lint check. @drenther and @anantoghosh thoughts on this?

[anantoghosh]

@skyshader If we are already going to use Travis CI, then I don't think we need to execute all tests for every commit.

[drenther]

@anantoghosh That's right. But I think only the repo owner can setup Travis CI hooks. Right?

[anantoghosh]

Yeah, the owner needs to do it.

[skyshader]

Yep makes sense. But I don't think that we as moderators have access to set up CI with the repositories. That's the reason I thought it would be better if tested code will only be committed. But yeah, lets not enforce it and mention it in Contributing guide to write and run the tests before submitting the PR. Thoughts? @anantoghosh and @drenther

[anantoghosh]

Would it be possible to ask Akshit sir, he just needs to enable the repo in travis. Rest we can do ourselves. @skyshader Ok

[drenther]

Yes, @anantoghosh @skyshader. If he just enables the repo in Travis and commits the travis.yml file, we will be set to go.

[anantoghosh]

This PR is good to merge. package-lock.json will mess up we will have to handle that separately.