Review Request 47641: Fix navbar alignment on large screen.

[Tomasz Janiszewski]

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https://reviews.apache.org/r/47641/
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Review request for mesos, Freddy Ayuso-Henson, haosdent huang, Tim Anderegg, 
and Vinod Kone.


Bugs: MESOS-3690
    https://issues.apache.org/jira/browse/MESOS-3690


Repository: mesos


Description
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On screens with more than 992px navbar is aligned to the bottom
of the header, this patch change this behaviour and navbar
is vertically centered


Diffs
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  site/source/assets/css/main.css bfee2dd6810bb341c1e8c2504d6338a76518ace9 

Diff: https://reviews.apache.org/r/47641/diff/


Testing
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![compare](http://s32.postimg.org/6soj2c6h1/compare.png)


Thanks,

Tomasz Janiszewski