Added screen reader text in summary and trend tooltips.
Merge pull request #102 from edx/dsjen/accessible-tooltips

Added screen reader text in summary and trend tooltips.
Updated Page Copy

- Updated copy according to feedback
- Removed demo engagement content since it is no longer useful
- Removed flag around engagement navbar item
Updated Graph/Chart Tooltips
Updated Metric Tooltips
Updated Engagement Table Column Ordering

Column ordering now matches that of the metrics and graph.
Updated Loading Message and Positioning
Updated Homepage Text
Removed Dead Code and Permanently Enabled Features

- The overview and performance demo pages don't even load. Time to die.
- The engagement features should always be active and therefore not behind a feature gate.