Unread and date separator layout improved (#319)

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@RocketChat/ReactNative

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- [x] Unread and date separator layout
- [x] "Start of conversation"/"Loading messages" label

![screen shot 2018-05-30 at 18 10 43](https://user-images.githubusercontent.com/804994/40747867-0424964a-6435-11e8-9293-31cc43c110ab.png)
![screen shot 2018-05-30 at 18 09 05](https://user-images.githubusercontent.com/804994/40747868-04484784-6435-11e8-8c31-92e0776276f0.png)



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