WIP: new layout header

[mipyykko]

Two-tier header: HY logo, (possible HY navigation), language switcher; MOOC logo and navigation below that.

Top header does not have any navigation currently -- semantically it should navigate inside the HY site, but we're not actually under that, so...? HY logo is also missing the link to the website.

The navigation on the mobile has changed to using a drawer instead of a dropdown. A possible multiple tier drawer is not implemented; does support nesting but not used for now. The menu should probably have more content. The profile/login/logout buttons on the navigation bar have not been touched yet.

• adds font(s) -- the current duplication for the same font is just to keep some parts that may use the variables working
• adds palette -- theme.palette.common.*
• adds some styling to the components on the theme level -- button first and foremost; this must still be revisited as most of the buttons below the header will be wrong
• adds some SVG icons copied from the HY design system