Improve performance of events processing

[pdpino]

• Move events sorting to WeekView, to only sort them once
  • An eventsByDate object is created, with date as keys (in format "YYYY-MM-DD") and list of events as values
  • Then each Events component accesses the events from the same object, avoiding extra copies and sorting more than necessary
  • The sorting is performed over the list of events from the same day, which should be much less than the number of total events
• Memoize functions in WeekView.render, to avoid re-calculations when not necessary
• Make Events and Header PureComponents, to avoid extra re-renders
• Create Times component to display time column (is a PureComponent to avoid extra re-renders)
• In WeekView.scrollEnded only update the state if the date actually changes, to avoid an extra re-render
• Other minor changes
  • Move variables in App.js to state, to ease testing process
  • Reuse calculateDaysArray() function

PS: in my use case, I have about 1500 events, and it was taking 3-5 seconds to load 😬
PS2: I realize these may be a lot of logic changes and you may not agree with them, but I needed to make these improvements for my use case ASAP, so I just applied them without discussing them first. I understand if you want to further improve them, or reject them at all 🙂

[hoangnm]

Impressive! Overall, everything is great, the logic to transform events array to object is neat and clean, this is really a better way. I only left one comment. Thanks a lot for the PR!