[lib] Fix edge case that can lead to non-contiguous message state
[lib] Fix edge case that can lead to non-contiguous message state

Summary:
By "non-continguous message state", I mean that if there are three messages (in order) A, B, C, we are only showing A and C.

We can get in this state if:

- The user has more than `defaultNumberPerThread` ([20](https://github.com/Ashoat/squadcal/blob/master/lib/types/message-types.js#L742)) new messages for a given thread
- The user presses a recent notification to open that thread, immediately inserting the message from the notification into the `messageStore` through a `SAVE_MESSAGES` Redux action

The current implementation of `isContiguous` would see that the message from the notification is already in the `messageStore`, and incorrectly conclude that the new messages are continguous with the existing messages.

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D101

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