Updated the links to use MARKETING_SITE_BASE_URL

[asadiqbal08]

fixes: mitodl/mitxonline#268

From the menu in MFE in MITx Online edX, there are a couple links that need to be updated.

• "Dashboard" menu item should link to: MARKETING_SITE_URL/dashboard rather than LMS_BASE_URL/dashboard/
• "Profile" menu item should link to: MARKETING_SITE/profile rather than LMS_BASE_URL/u/USERNAME
• Also, in the menu in the 'legacy' theme, clicking on the username will take you to LMS_BASE_URL/dashboard, but should take you to MARKETING_SITE/dashboard

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