Update header implementation for upcoming Discourse changes

[davidtaylorhq]

Checklist:

• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

Closes #116

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Discourse will soon be dropping the 'widget' system for rendering its header. This commit switches the theme from the deprecated APIs to the their modern equivalents. As a nice side effect, this also fixes the layout issue of the 'visit the curriculum' button for anonymous users.

More information on the changes can be found at https://meta.discourse.org/t/296544

Before:

After:

[ShaunSHamilton]

Thank you for taking the time to update this. I might be incorrectly testing this, but this is what I see:

The header button is still too far up (on Chrome)