Engagement trend view and table displays engagement activities.

  * Refactored enrollment-trend-view to trend-view that displays multiple trends.
  * Using activities call on the api-client instead of deprecated recent_activity call.
  * Forum activities for trends put behind feature gate.
Merge pull request #92 from edx/dsjen/multi-trend-line

Engagement trend view and table displays engagement activities.
Updated Enrollment Trend Styling