WelcomePage: Redesign to center the contents, other misc improvements

We now remove the header from the page, and replace it with a separator (it still lives with an InlineMessage for error handling.) The contents of this page are now centered, and the maximum width of the buttons are reduced.

Along with that are two smaller misc improvements. One is that the duplicate separator underneath "Register" is now gone. Another is that the full settings page may now be opened from here, allowing users to access more than proxy settings.

Before

After (Error)

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After (Logged Out)

After (Logged In)