Mods to Event Generator and limit provider

- Changed SEVERE to use `is-event--purple` style.
- Mods to EventTelemetryProvider.js:
  - Adds a more random start time to each event.
  - Reduces frequency at which a severity is applied to events.

example/eventGenerator/EventLimitProvider.js

@@ -25,7 +25,7 @@ export const SEVERITY_CSS = {
   WARNING: 'is-event--yellow',
   DISTRESS: 'is-event--red',
   CRITICAL: 'is-event--red',
-  SEVERE: 'is-event--red'
+  SEVERE: 'is-event--purple'
 };
 
 const NOMINAL_SEVERITY = {