implemented the new design navbar

[ShravaniAK]

current navbar :

I have changed the following,

1. added border-radius to the search,
2. added border-radius to the corners of the tabs items,
3. Shifted the git source icons to the left

[knative-prow]

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[Cali0707]

/cc @mmejia02 @zainabhusain227

Can the two of you check the preview and make sure it matches the designs?

[knative-prow-robot]

PR needs rebase.

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[zainabhusain227]

Hey @ShravaniAK, where is the knative version number? I don't see it in the revised version. Besides that the rounded corners look good to me!