Move QC thresholding to backend

[shihabdider]

Is your feature request related to a problem? Please describe.
Currently the front-end is computing the quality classes for each case. This should really be done in the backend and passed along with the metadata. The front-end should only handle how that information is displayed (color, font, etc.) not actually compute it.

Describe the solution you'd like
The thresholding method currently implemented in JavaScript should be moved into an R method on the backend (in the Skilift package). The metadata.json will then contain the appropriate fields for display on the front-end. The front-end will control how this information is displayed.

A tentative schema for the metadata.json object:

{
  "quality": {
    "class": "good|poor|NA",
    "reasons": [
      "reason1",
      "reason2"
    ]
  }
}

Where reason1, reason2 etc. will be the strings that are displayed in the onhover dropdown.

@shihabdider will provide the backend implementation and update the metadata.jsons. @xanthopoulakis will update his front-end code to interface with this new object (please wait until @shihabdider is finished updating the metadata.json to avoid breaking changes to running instances).