GH-2236: Update onboarding branding

[leuryr]

Changes copy and logos in order to implement the new branding for the browser and search. Though the install loading modal screen is not implemented yet, I added the text for the heading, in messages.json, where the copy change is supposed to go.

Also made some small CSS changes to pull certain things more in line with the Zeplin designs.

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[wlycdgr]

LGTM for merging into onboarding branch