Group Activity - MRs created Chart w/ Single Series: Add configurable column chart

The first version of the this generic reports page has the following requirements:

1. The first report page will be linked from Group Recent Activity and display a column chart showing merge requests open. The design issue provides a more complete picture. This issue gets us started.

2. The date range is set to the last ninety days and no date selector is shown.

3. The columns will be grouped by week. No selector for grouping by days or months is included.

4. The chart will display one one series. Our API we create should leave room for multi-series charts which are the most common use case.

5. No data table will be shown below the chart.