13 countries table rebase

[tjanowsk]

Rebased Duncan's branch and opened it as a PR

Changes:

• added ag-grid to the main page
• wired the component to the existing api
• simplified the api classes - it is called only once and fetches all the summary data in one go; we can always add complications later if necessary
• changed the default sorting to descending by number of infected, so USA can be number 1

closes #13