Bye bye robot.http, hello fetch

[KeesCBakker]

We've been talking for a while now to move away from the robot.http implemention of the HTTP client. I did some more refactoring and we finally don't have to use it anymore. This means our bot is a bit cleaner.

Main points:

• deleted http.js -- which could be considered a breaking change from the previous package
• rewrite of RocketChatUploader.js -- but I fear it has no tests, so I have no way of knowing if we're OK
  
• small rewrite for GrafanaClient -- we dropped createHttpClient and so better validation of the Grafana response.
• added sendDashboardChartFromString for convenience to the Bot (should be another PR, but makes testing easier for me).
• added support for mocha testing to the dev container with the hbenl.vscode-mocha-test-adapter test adapter. Now we don't have to do a .only all the time, but just run the test from the IDE:
  

[KeesCBakker]

Will test this tomorrow with a real Hubot. But I only use the combination of Slack & S3. @stephenyeargin to you have some sort of a RocketChat instance?

[stephenyeargin]

I do not, that was introduced by @mfilotto back in #70. I would be fine with a good faith effort, and we can address bugs if they come in.

[KeesCBakker]

Refactored the mocha tests, swapped out mocha.opts in favor of .mocharc.json. All tests now use the same config.

Check how it looks in a Visual Studio Code dev container with the Mocha Test adapter:

This will make it easier to run certain tests directly from your IDE.