Better guarantees and documentation for events

[breezykermo]

Because event IDs exist in sheet-like data sources outside of timemap, and the appropriate API endpoints for timemap to consume are usually created by datasheet-server, there is no way in timemap itself to enforce the requirement that event IDs are consistent between data updates. See this issue thread for greater context. This should be made clearer in the documentation.

There can and should, however, be validation of event data to ensure that IDs are unique. This should be done in validators.js, appropriately dropping any events with duplicate IDs (as we currently do with filters). It's possible that timemap behaves reasonably when there are events with the same ID, but this is not a case that I can imagine a use case for.