Add Zooniverse logo to header

[seanmiller26]

On final implementation, after ERAS stats are incorporated, the logo should be clickable and take volunteers to their homepage. (homepage issue linked here shortly.)

Logo can be found here:
https://github.com/zooniverse/Brand

Size should match the navigation menu. Margin of 30 px on the left

User story:
As a volunteer,
When I click the Zooniverse logo
The app should take me to the homepage
So that I can see my total classifications, etc.