feat(Masthead): add support for megamenu in masthead l1 (#4099)

### Related Ticket(s)

React: Change Masthead L1 from behind the feature flag #4010

### Description

Add support for megamenu in Masthead L1

<img width="1438" alt="Screen Shot 2020-09-30 at 5 42 19 PM" src="https://user-images.githubusercontent.com/54281166/94742861-6a3d6500-0344-11eb-9bc0-389c09f636b0.png">


### Changelog

**New**

- add MegaMenu to the `<MastheadL1 />` 


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