Framework: Remove Immutable

[spen]

I had started working on removing ImmutableJS some time ago and just got around to neatening up that work.
With Immutable gone, we should see smaller bundles (one less dependency) and we should find it makes it a little easier to remove flux from the areas that current use Immutable.

From what I understand, the only steps needed to remove Immutable would be:

• Refactor client/lib/invites/ to use regular JS.
• Refactor client/lib/notification-settings-store to use regular JS.
  • Refactor toggleState to use regular JS. (Framework: Convert NotificationsSettings store to redux & remove Immutable from module. #28653)
  • Refactor index to use regular JS.
  • Change all propTypes to regular JS types.
• Remove dependency.
• Update references in docs & comments.

Relates to: #19825

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