Re-structure 'rankings.json'

[pmsoltani]

Bug description

The metric section of the rankings.json file is flat regarding the ranking system. Therefore if there are metrics with the same name in different rankings, we cannot distinguish them; consider:

THE ranking:

"International Students": { "name": "% International Students", "type": "percent" }

QS ranking:

"International Students": { "name": "International Students Score", "type": "percent" }

Both metrics are called "International Students", but one is a real percentage value, reflecting an actual number (THE) and the other is just a score (from 0 to 100) obtained by comparing a value across different institutions (QS).

In the current json file, we cannot have this, because the two metrics have the same name and so we need to change its structure.

Suggested solution

Envelope the metrics of each ranking in its ranking_system (qs, shanghai, the), as each metric occurs only once in each ranking.