Remove observer in favor of didReceiveAttrs
Remove observer in favor of didReceiveAttrs

This commit removes the observer in the component, and uses
`didReceiveAttrs` instead to update chart data when `content` changes.

Also corrects a few minor formatting issues.

Caveat: This removes support for Ember versions < 1.13.x (pre-glimmer),
and should be marked as a breaking change.

supersabillon/ember-highcharts,supersabillon/ember-highcharts,trabus/ember-highcharts,ahmadsoe/ember-highcharts,gregjopa/ember-highcharts,CubiclDev/ember-highcharts,trabus/ember-highcharts,CubiclDev/ember-highcharts,ahmadsoe/ember-highcharts,trabus/ember-highcharts,CubiclDev/ember-highcharts,ahmadsoe/ember-highcharts,supersabillon/ember-highcharts,gregjopa/ember-highcharts,gregjopa/ember-highcharts